Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Activities:
Net Income	$ 401	$ 521	$ 730
Adjustments to reconcile net income to net cash flows provided by operating activities:
Non-cash interest and operating expenses	34	36	26
Net amortization of premium (discount) on investments	(35)	(31)	(46)
Provision (benefit) for deferred income taxes	65	74	250
Net realized investment losses (gains)	(22)	32	(40)
Bargain purchase gain and settlement of pre-existing relationships	0	0	(58)
Change in premiums receivable, net of premiums and commissions payable	(388)	(6)	(69)
Change in ceded unearned premium reserve	20	58	90
Change in unearned premium reserve	224	39	(424)
Change in loss and loss adjustment expense reserve, net	(528)	(173)	142
Change in financial guaranty variable interest entities' assets and liabilities, net	(27)	(5)	(15)
Change in credit derivative assets and liabilities, net	(1)	(62)	(144)
Purchases of securities	(267)	0	0
Other changes in investment vehicles	0	(21)	(9)
Other changes in investment vehicles	15	0	0
Net cash flows provided by (used in) operating activities	(509)	462	433
Fixed-maturity securities:
Purchases	(873)	(1,881)	(2,552)
Sales	1,805	1,180	1,701
Maturities and paydowns	781	962	821
Short-term investments with maturities of over three months:
Purchases	(229)	(243)	(255)
Sales	2	23	102
Maturities and paydowns	316	207	191
Net sales (purchases) of short-term investments with original maturities of less than three months	(623)	(84)	36
Net proceeds from paydowns on financial guaranty variable interest entities’ assets	139	116	147
Net proceeds from sales of financial guaranty variable interest entities’ assets	51	0	0
Acquisitions, net of cash acquired (see Note 2)	(145)	0	95
Proceeds from maturity of other invested asset	0	0	85
Proceeds from sales of other invested assets	36	38	2
Purchases of other invested assets	(88)	(20)	(23)
Other	(3)	(1)	(5)
Net cash flows provided by (used in) investing activities	1,169	297	345
Financing activities
Dividends paid	(74)	(71)	(70)
Repurchases of common stock	(500)	(500)	(501)
Net paydowns of financial guaranty variable interest entities’ liabilities	(181)	(116)	(157)
Paydown of long-term debt	(4)	(101)	(30)
Other	(15)	(7)	(8)
Proceeds from issuance of collateralized loan obligations	482	0	0
Repayment of warehouse loans and equity	(306)	0	0
Contributions from noncontrolling interests to investment vehicles	18	0	0
Distributions to redeemable noncontrolling interests from investment vehicles	(4)	0	0
Net cash flows provided by (used in) financing activities	(584)	(795)	(766)
Effect of foreign exchange rate changes	3	(4)	5
Increase (decrease) in cash and restricted cash	79	(40)	17
Cash and restricted cash at beginning of period	104	144	127
Cash and restricted cash at end of period	183	104	144
Supplemental cash flow information
Income taxes	4	(4)	10
Interest on long-term debt	84	99	77
Supplemental disclosure of non-cash investing and financing activities:
Purchases of fixed-maturity investments	(188)	(4)	(32)
Sales of fixed-maturity investments	44	0	0
Reconciliation of Cash and Restricted Cash to Consolidate Balance Sheet [Abstract]
Cash and restricted cash at the end of period	$ 183	$ 144	$ 144