Insurance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statutory Accounting Practices [Line Items]
Cash and cash equivalents	$ 162	$ 99
Marketable securities	449	475
Total cash, cash equivalents, and marketable securities	611	574
Assets Held by Insurance Regulators	12	12
Prepaid reinsurance premiums and other reinsurance recoverables	611	546
Related party transaction, prepaid reinsurance premiums and other reinsurance recoverables	95	91
Earned insurance premiums	154	133	$ 90
Insurance loss and loss adjustment expenses	63	55	36
Unearned insurance premiums and fees	723	650
Related Party Transactions Unearned Premiums and Fees	621	556
Premiums Written and Earned [Abstract]
Direct premiums written	385	371	328
Assumed premiums written	0	0	0
Ceded premiums written	(227)	(215)	(194)
Net premiums written	158	156	134
Direct premiums earned	320	298	254
Assumed premiums earned	0	0	0
Ceded premiums earned	(162)	(142)	(121)
Premiums Earned, Net	158	156	133
Insurance Expenses [Abstract]
Insurance losses	143	146	80
Loss adjustment expenses	8	5	5
Reinsurance Income and Other Expenses, Net	(27)	(26)	(16)
Insurance expenses	124	125	69
Liability for reported insurance claims and estimate of unreported claims	8	6
Ceded insurance expenses	$ 104	$ 95	$ 76