Claim and Claim Adjustment Expense Reserves - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2010
Liability for Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses net of reinsurance	$ 34,000,000	$ 36,000,000
Loss Portfolio Transfer
Liability for Claims and Claims Adjustment Expense [Line Items]
Net A&EP claim and allocated claim adjustment expense reserves ceded to NICO				$ 1,600,000,000
Aggregate limit under A&EP Loss Portfolio Transfer				4,000,000,000
A&EP claim and allocated claim adjustment expense reserves ceded under existing third party reinsurance contracts transferred to NICO under A&EP Loss Portfolio Transfer				1,200,000,000
Reinsurance premium paid to NICO under A&EP Loss Portfolio Transfer				2,000,000,000
Net reinsurance receivables transferred to NICO under A&EP Loss Portfolio Transfer				215,000,000
Total consideration paid to NICO under AEP Loss Portfolio Transfer				$ 2,200,000,000
Net A&EP adverse development before consideration of LPT	60,000,000	$ 200,000,000
Cumulative amounts ceded under AEP Loss Portfolio Transfer	2,900,000,000		$ 2,800,000,000
Deferred reinsurance benefit yet to be recognized	354,000,000		334,000,000
Fair value of collateral trust account established by NICO under A&EP Loss Portfolio Transfer	$ 2,700,000,000		$ 2,800,000,000