Condensed Consolidated Statements of Cash Flows (unaudited) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Statement of Cash Flows [Abstract]
Net cash flows provided by (used in) operating activities	$ (332)	$ 27
Fixed-maturity securities:
Purchases	(196)	(411)
Sales	471	409
Maturities	177	225
Short-term investments with original maturities of over three months:
Purchases	(107)	(47)
Sales	2	0
Maturities	67	45
Net sales (purchases) of short-term investments with original maturities of less than three months	25	(114)
Net proceeds from paydowns on financial guaranty variable interest entities’ assets	26	33
Other	27	(14)
Net cash flows provided by (used in) investing activities	492	126
Financing activities
Dividends paid	(20)	(18)
Repurchases of common stock	(80)	(100)
Repurchases of common stock to pay withholding taxes	(15)	(12)
Net paydowns of financial guaranty variable interest entities’ liabilities	(25)	(33)
Paydown of long-term debt	(3)	(19)
Proceeds from option exercises	1	1
Net cash flows provided by (used in) financing activities	(142)	(181)
Effect of foreign exchange rate changes	1	1
Increase (decrease) in cash and restricted cash	19	(27)
Cash and restricted cash at beginning of period (see Note 9)	104	144	$ 144
Cash and restricted cash at end of period (see Note 9)	123	117	$ 104
Supplemental cash flow information
Income taxes	0	10
Interest on long-term debt	$ 9	$ 21