Summary of Significant Accounting Policies - Recent Accounting Pronouncements (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Distributions from partnership investments	[1]	$ 45	$ 27
Net cash provided by (used in) investing activities	[1]	$ (117)	(360)
Accounting Standards Update 2016-15 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Requirements and effects of new guidance		Specifies how certain transactions should be classified in the statement of cash flows. While the standard addresses multiple types of transactions, only a change in the treatment of distributions from equity method investments impacted the Company. Effects of adoption: using the nature of distribution approach, the Company reported $45 million of cash receipts related to distributions from partnership earnings in operating activities for the three months ended March 31, 2017. The Company reclassified $27 million for the three months ended March 31, 2016 from investing to operating activities in the Consolidated Statements of Cash Flows.
Distributions from partnership investments		$ 45
Accounting Standards Update 2016-15 [Member] | Restatement Adjustment [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Distributions from partnership investments			27
Net cash provided by (used in) investing activities			$ (27)
Accounting Standards Update 2017-07 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Requirements and effects of new guidance		Requires employers to separate the service cost component from the other components of net benefit cost. Under the new guidance, only service cost is eligible for capitalization (either deferred policy acquisition costs or capitalized software). The change in the capitalization rule is to be applied prospectively upon adoption. In addition, the income statement caption(s) where each component of net benefit cost is presented must be disclosed. Expected effects: the Company expects the effect of this new guidance to be immaterial to its results of operations.
Material effect on Company's financial statements		No
Accounting Standards Update 2016-01 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Requirements and effects of new guidance		Requires: • Entities to measure equity investments at fair value in net income if they are neither consolidated nor accounted for under the equity method • Cumulative effect adjustment to the beginning balance of retained earnings at adoption Expected effects: • Certain limited partnership interests carried at cost of $240 million as of March 31, 2017 will be reported at fair value at adoption • An increase to retained earnings of approximately $50 million, after-tax, if implemented as of March 31, 2017. Actual cumulative effect adjustment will depend on investments held and market conditions at adoption.
Current carrying value of certain limited partnership interests to be reported at fair value upon adoption of ASU 2016-01		$ 240
Impact of new guidance on equity if adopted at reporting date		$ 50
Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Requirements and effects of new guidance		Requires: • Companies to estimate and allocate the expected customer contract revenues among distinct goods or services based on relative standalone selling prices • Revenues to be recognized as goods or services are delivered • Extensive new disclosures including the presentation of additional categories of revenues and information about related contract assets and liabilities • Adoption through retrospective restatement with or without using certain practical expedients or adoption with a cumulative effect adjustment Expected effects: • Applies to the Company’s non-insurance, administrative service contracts but does not apply to certain contracts within the scope of other GAAP, such as insurance contracts • The Company currently expects to adopt the new guidance as of January 1, 2018 through retrospective restatement • The Company does not currently expect the adoption of the new guidance to have a material impact to its pattern of revenue recognition or net income • The Company is continuing to evaluate the new requirements. Specifically, the Company is evaluating the combination of contract guidance for certain customers when the Company provides both insurance and non-insurance products, the deferral of revenue for services provided after the termination of certain administrative contracts and the Company’s status as principal or agent for certain performance obligations.
Material effect on Company's financial statements		No

[1] As required in adopting Accounting Standard Update ("ASU") 2016-15, the Company retrospectively reclassified $27 million of cash distributions from partnership earnings from investing to operating activities for the first quarter of 2016. The comparable amount reported in operating activities in 2017 was $45 million. See Note 2 for further discussion.