Claim and Claim Adjustment Expense Reserves - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2010
Liability for Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses net of reinsurance	$ 269,000,000	$ 16,000,000	$ 342,000,000	$ 137,000,000
Reinsurance reinstatement premium	6,000,000		6,000,000
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	0	$ 0	60,000,000	$ 200,000,000
Cumulative amounts ceded under AEP Loss Portfolio Transfer	2,900,000,000		2,900,000,000		$ 2,800,000,000
Deferred reinsurance benefit yet to be recognized	334,000,000		334,000,000		334,000,000
Fair value of collateral trust account established by NICO under A&EP Loss Portfolio Transfer	2,900,000,000		2,900,000,000		$ 2,800,000,000
Hurricane Harvey
Liability for Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses	149,000,000		149,000,000
Hurricane Irma
Liability for Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses	95,000,000		95,000,000
Hurricane Maria
Liability for Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses	$ 20,000,000		$ 20,000,000