Condensed Consolidating Financial Information - Cash Flow Statements (Details) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Sep. 30, 2018	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities				$ 3,770	$ 4,086	$ 4,026
Cash Flows from Investing Activities
Net change in loans due to (from) affiliates				0	0	0
Proceeds from investments sold:
Fixed maturities and equity securities				2,655	2,012	1,544
Investment maturities and repayments:
Fixed maturities and equity securities				2,151	2,051	1,755
Commercial mortgage loans				215	335	316
Other sales, maturities and repayments (primarily short-term and other long-term investments)				734	1,702	1,431
Investments purchased or originated:
Fixed maturities and equity securities				(5,637)	(5,628)	(5,191)
Commercial mortgage loans				(312)	(430)	(165)
Other (primarily short-term and other long-term investments)				(1,189)	(1,065)	(1,698)
Property and equipment purchases, net				(528)	(471)	(461)
Acquisitions, net of cash acquired				(24,455)	(209)	(4)
Other, net				(12)	0	(101)
Net cash provided by (used in) investing activities				(26,378)	(1,703)	(2,574)
Cash Flows from Financing Activities
Net change in amounts due to (from) affiliates				0	0	0
Intercompany dividends paid				0	0	0
Deposits and interest credited to contractholder deposit funds				1,040	1,230	1,460
Withdrawals and benefit payments from contractholder deposit funds				(1,151)	(1,363)	(1,362)
Net change in short-term debt				1,487	80	(148)
Payments for debt extinguishment				0	(313)	0
Repayment of long-term debt		$ (131)	$ (250)	(131)	(1,250)	0
Net proceeds on issuance of long-term debt	$ 20,000			22,856	1,581	0
Repurchase of common stock				(342)	(2,725)	(139)
Issuance of common stock				68	131	36
Other, net				(312)	(22)	(72)
Net cash provided by (used in) financing activities				23,515	(2,651)	(225)
Effect of foreign currency rate changes on cash and cash equivalents				(24)	55	(10)
Net increase in cash and cash equivalents				883	(213)	1,217
Cash and cash equivalents, January 1,		2,972	3,185	2,972	3,185	1,968
Cash and cash equivalents, December 31,				3,855	2,972	3,185
Consolidation, Eliminations [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities				(2,518)	(758)	(580)
Cash Flows from Investing Activities
Net change in loans due to (from) affiliates				5,733	1,955	(78)
Proceeds from investments sold:
Fixed maturities and equity securities				0	0	0
Investment maturities and repayments:
Fixed maturities and equity securities				0	0	0
Commercial mortgage loans				0	0	0
Other sales, maturities and repayments (primarily short-term and other long-term investments)				0	0	0
Investments purchased or originated:
Fixed maturities and equity securities				0	0	0
Commercial mortgage loans				0	0	0
Other (primarily short-term and other long-term investments)				0	0	0
Property and equipment purchases, net				0	0	0
Acquisitions, net of cash acquired				0	0	0
Other, net				0	0	0
Net cash provided by (used in) investing activities				5,733	1,955	(78)
Cash Flows from Financing Activities
Net change in amounts due to (from) affiliates				(5,733)	(1,955)	78
Intercompany dividends paid				2,518	758	580
Deposits and interest credited to contractholder deposit funds				0	0	0
Withdrawals and benefit payments from contractholder deposit funds				0	0	0
Net change in short-term debt				0	0	0
Payments for debt extinguishment				0	0	0
Repayment of long-term debt				0	0	0
Net proceeds on issuance of long-term debt				0	0	0
Repurchase of common stock				0	0	0
Issuance of common stock				0	0	0
Other, net				0	0	0
Net cash provided by (used in) financing activities				(3,215)	(1,197)	658
Effect of foreign currency rate changes on cash and cash equivalents				0	0	0
Net increase in cash and cash equivalents				0	0	0
Cash and cash equivalents, January 1,		0	0	0	0	0
Cash and cash equivalents, December 31,				0	0	0
Cigna [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities				145
Investments purchased or originated:
Other (primarily short-term and other long-term investments)				0
Other, net				(27,115)
Net cash provided by (used in) investing activities				(27,115)
Cash Flows from Financing Activities
Net change in amounts due to (from) affiliates				4,437
Net change in short-term debt				0
Payments for debt extinguishment				0
Repayment of long-term debt				0
Net proceeds on issuance of long-term debt				22,856
Repurchase of common stock				(32)
Issuance of common stock				1
Net cash provided by (used in) financing activities				27,213
Net increase in cash and cash equivalents				243
Cash and cash equivalents, December 31,				243
Cigna [Member] | Reportable Legal Entities [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities				145	0	0
Cash Flows from Investing Activities
Net change in loans due to (from) affiliates				0	0	0
Proceeds from investments sold:
Fixed maturities and equity securities				0	0	0
Investment maturities and repayments:
Fixed maturities and equity securities				0	0	0
Commercial mortgage loans				0	0	0
Other sales, maturities and repayments (primarily short-term and other long-term investments)				0	0	0
Investments purchased or originated:
Fixed maturities and equity securities				0	0	0
Commercial mortgage loans				0	0	0
Other (primarily short-term and other long-term investments)				0	0	0
Property and equipment purchases, net				0	0	0
Acquisitions, net of cash acquired				(27,115)	0	0
Other, net				0	0	0
Net cash provided by (used in) investing activities				(27,115)	0	0
Cash Flows from Financing Activities
Net change in amounts due to (from) affiliates				4,437	0	0
Intercompany dividends paid				0	0	0
Deposits and interest credited to contractholder deposit funds				0	0	0
Withdrawals and benefit payments from contractholder deposit funds				0	0	0
Net change in short-term debt				0	0	0
Payments for debt extinguishment				0	0	0
Repayment of long-term debt				0	0	0
Net proceeds on issuance of long-term debt				22,856	0	0
Repurchase of common stock				(32)	0	0
Issuance of common stock				1	0	0
Other, net				(49)	0	0
Net cash provided by (used in) financing activities				27,213	0	0
Effect of foreign currency rate changes on cash and cash equivalents				0	0	0
Net increase in cash and cash equivalents				243	0	0
Cash and cash equivalents, January 1,		0	0	0	0
Cash and cash equivalents, December 31,				243	0	0
Old Cigna [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities					602	376
Investments purchased or originated:
Other (primarily short-term and other long-term investments)					(6)	(3)
Other, net					(11)	(8)
Net cash provided by (used in) investing activities					(17)	(11)
Cash Flows from Financing Activities
Net change in amounts due to (from) affiliates					1,955	(78)
Net change in short-term debt					100	(100)
Payments for debt extinguishment					(313)	0
Repayment of long-term debt					(1,250)	0
Net proceeds on issuance of long-term debt					1,581	0
Repurchase of common stock					(2,725)	(139)
Issuance of common stock					131	36
Net cash provided by (used in) financing activities					(594)	(363)
Net increase in cash and cash equivalents					(9)	2
Cash and cash equivalents, January 1,		9	18	9	18	16
Cash and cash equivalents, December 31,					9	18
Old Cigna [Member] | Reportable Legal Entities [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities				2,416	602	376
Cash Flows from Investing Activities
Net change in loans due to (from) affiliates				(4,412)	0	0
Proceeds from investments sold:
Fixed maturities and equity securities				0	0	0
Investment maturities and repayments:
Fixed maturities and equity securities				0	0	0
Commercial mortgage loans				0	0	0
Other sales, maturities and repayments (primarily short-term and other long-term investments)				63	0	0
Investments purchased or originated:
Fixed maturities and equity securities				(10)	0	0
Commercial mortgage loans				0	0	0
Other (primarily short-term and other long-term investments)				0	(6)	(3)
Property and equipment purchases, net				0	0	0
Acquisitions, net of cash acquired				0	0	0
Other, net				0	(11)	(8)
Net cash provided by (used in) investing activities				(4,359)	(17)	(11)
Cash Flows from Financing Activities
Net change in amounts due to (from) affiliates				1,121	1,955	(78)
Intercompany dividends paid				0	0	0
Deposits and interest credited to contractholder deposit funds				0	0	0
Withdrawals and benefit payments from contractholder deposit funds				0	0	0
Net change in short-term debt				1,400	100	(100)
Payments for debt extinguishment				0	(313)	0
Repayment of long-term debt				(131)	(1,250)	0
Net proceeds on issuance of long-term debt				0	1,581	0
Repurchase of common stock				(310)	(2,725)	(139)
Issuance of common stock				67	131	36
Other, net				(213)	(73)	(82)
Net cash provided by (used in) financing activities				1,934	(594)	(363)
Effect of foreign currency rate changes on cash and cash equivalents				0	0	0
Net increase in cash and cash equivalents				(9)	(9)	2
Cash and cash equivalents, January 1,		9	18	9	18	16
Cash and cash equivalents, December 31,				0	9	18
Express Scripts Holding Company [Member] | Reportable Legal Entities [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities				(36)	0	0
Cash Flows from Investing Activities
Net change in loans due to (from) affiliates				(200)	0	0
Proceeds from investments sold:
Fixed maturities and equity securities				0	0	0
Investment maturities and repayments:
Fixed maturities and equity securities				0	0	0
Commercial mortgage loans				0	0	0
Other sales, maturities and repayments (primarily short-term and other long-term investments)				0	0	0
Investments purchased or originated:
Fixed maturities and equity securities				0	0	0
Commercial mortgage loans				0	0	0
Other (primarily short-term and other long-term investments)				0	0	0
Property and equipment purchases, net				0	0	0
Acquisitions, net of cash acquired				1,676	0	0
Other, net				0	0	0
Net cash provided by (used in) investing activities				1,476	0	0
Cash Flows from Financing Activities
Net change in amounts due to (from) affiliates				(807)	0	0
Intercompany dividends paid				0	0	0
Deposits and interest credited to contractholder deposit funds				0	0	0
Withdrawals and benefit payments from contractholder deposit funds				0	0	0
Net change in short-term debt				0	0	0
Payments for debt extinguishment				0	0	0
Repayment of long-term debt				0	0	0
Net proceeds on issuance of long-term debt				0	0	0
Repurchase of common stock				0	0	0
Issuance of common stock				0	0	0
Other, net				0	0	0
Net cash provided by (used in) financing activities				(807)	0	0
Effect of foreign currency rate changes on cash and cash equivalents				0	0	0
Net increase in cash and cash equivalents				633	0	0
Cash and cash equivalents, January 1,		0	0	0	0	0
Cash and cash equivalents, December 31,				633	0	0
Express Scripts Incorporated [Member] | Reportable Legal Entities [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities				80	0	0
Cash Flows from Investing Activities
Net change in loans due to (from) affiliates				0	0	0
Proceeds from investments sold:
Fixed maturities and equity securities				0	0	0
Investment maturities and repayments:
Fixed maturities and equity securities				0	0	0
Commercial mortgage loans				0	0	0
Other sales, maturities and repayments (primarily short-term and other long-term investments)				0	0	0
Investments purchased or originated:
Fixed maturities and equity securities				0	0	0
Commercial mortgage loans				0	0	0
Other (primarily short-term and other long-term investments)				0	0	0
Property and equipment purchases, net				(6)	0	0
Acquisitions, net of cash acquired				23	0	0
Other, net				0	0	0
Net cash provided by (used in) investing activities				17	0	0
Cash Flows from Financing Activities
Net change in amounts due to (from) affiliates				(54)	0	0
Intercompany dividends paid				0	0	0
Deposits and interest credited to contractholder deposit funds				0	0	0
Withdrawals and benefit payments from contractholder deposit funds				0	0	0
Net change in short-term debt				0	0	0
Payments for debt extinguishment				0	0	0
Repayment of long-term debt				0	0	0
Net proceeds on issuance of long-term debt				0	0	0
Repurchase of common stock				0	0	0
Issuance of common stock				0	0	0
Other, net				0	0	0
Net cash provided by (used in) financing activities				(54)	0	0
Effect of foreign currency rate changes on cash and cash equivalents				0	0	0
Net increase in cash and cash equivalents				43	0	0
Cash and cash equivalents, January 1,		0	0	0	0	0
Cash and cash equivalents, December 31,				43	0	0
Medco Health Solutions Incorporated [Member] | Reportable Legal Entities [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities				(304)	0	0
Cash Flows from Investing Activities
Net change in loans due to (from) affiliates				0	0	0
Proceeds from investments sold:
Fixed maturities and equity securities				0	0	0
Investment maturities and repayments:
Fixed maturities and equity securities				0	0	0
Commercial mortgage loans				0	0	0
Other sales, maturities and repayments (primarily short-term and other long-term investments)				0	0	0
Investments purchased or originated:
Fixed maturities and equity securities				0	0	0
Commercial mortgage loans				0	0	0
Other (primarily short-term and other long-term investments)				0	0	0
Property and equipment purchases, net				0	0	0
Acquisitions, net of cash acquired				0	0	0
Other, net				0	0	0
Net cash provided by (used in) investing activities				0	0	0
Cash Flows from Financing Activities
Net change in amounts due to (from) affiliates				304	0	0
Intercompany dividends paid				0	0	0
Deposits and interest credited to contractholder deposit funds				0	0	0
Withdrawals and benefit payments from contractholder deposit funds				0	0	0
Net change in short-term debt				0	0	0
Payments for debt extinguishment				0	0	0
Repayment of long-term debt				0	0	0
Net proceeds on issuance of long-term debt				0	0	0
Repurchase of common stock				0	0	0
Issuance of common stock				0	0	0
Other, net				0	0	0
Net cash provided by (used in) financing activities				304	0	0
Effect of foreign currency rate changes on cash and cash equivalents				0	0	0
Net increase in cash and cash equivalents				0	0	0
Cash and cash equivalents, January 1,		0	0	0	0	0
Cash and cash equivalents, December 31,				0	0	0
Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities				3,987	4,242	4,230
Cash Flows from Investing Activities
Net change in loans due to (from) affiliates				(1,121)	(1,955)	78
Proceeds from investments sold:
Fixed maturities and equity securities				2,655	2,012	1,544
Investment maturities and repayments:
Fixed maturities and equity securities				2,151	2,051	1,755
Commercial mortgage loans				215	335	316
Other sales, maturities and repayments (primarily short-term and other long-term investments)				671	1,702	1,431
Investments purchased or originated:
Fixed maturities and equity securities				(5,627)	(5,628)	(5,191)
Commercial mortgage loans				(312)	(430)	(165)
Other (primarily short-term and other long-term investments)				(1,189)	(1,059)	(1,695)
Property and equipment purchases, net				(522)	(471)	(461)
Acquisitions, net of cash acquired				961	(209)	(4)
Other, net				(12)	11	(93)
Net cash provided by (used in) investing activities				(2,130)	(3,641)	(2,485)
Cash Flows from Financing Activities
Net change in amounts due to (from) affiliates				732	0	0
Intercompany dividends paid				(2,518)	(758)	(580)
Deposits and interest credited to contractholder deposit funds				1,040	1,230	1,460
Withdrawals and benefit payments from contractholder deposit funds				(1,151)	(1,363)	(1,362)
Net change in short-term debt				87	(20)	(48)
Payments for debt extinguishment				0	0	0
Repayment of long-term debt				0	0	0
Net proceeds on issuance of long-term debt				0	0	0
Repurchase of common stock				0	0	0
Issuance of common stock				0	0	0
Other, net				(50)	51	10
Net cash provided by (used in) financing activities				(1,860)	(860)	(520)
Effect of foreign currency rate changes on cash and cash equivalents				(24)	55	(10)
Net increase in cash and cash equivalents				(27)	(204)	1,215
Cash and cash equivalents, January 1,		$ 2,963	$ 3,167	2,963	3,167	1,952
Cash and cash equivalents, December 31,				$ 2,936	$ 2,963	$ 3,167