Investments (details) - Changes in Net Unrealized Investment Gains - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in net unrealized investment gains [Roll Forward]
Changes in net unrealized investment gains	$ 522	$ (887)	$ (1,429)
Income tax expense (benefit)	110	(289)	(392)
Change in net unrealized gain on investment securities, net of tax	412	(598)	(1,037)
Balance, beginning of year	(755)
Balance, end of year	(343)	(755)
Accumulated Other Comprehensive Income (Loss) [Member]
Changes in net unrealized investment gains [Roll Forward]
Changes in net unrealized investment gains	522	(887)	(1,429)
Income tax expense (benefit)	110	(289)	(392)
Change in net unrealized gain on investment securities, net of tax	412	(598)	(1,037)
Changes in Net Unrealized Gains on Investment Securities [Member]
Changes in net unrealized investment gains [Roll Forward]
Changes in net unrealized investment gains	302	(862)	(1,034)
Income tax expense (benefit)	78	(303)	(357)
Change in net unrealized gain on investment securities, net of tax	224	(559)	(677)
Balance, beginning of year	730	1,289	1,966
Balance, end of year	954	730	1,289
Changes in Net Unrealized Gains on Investment Securities [Member] | Fixed maturities [Member]
Changes in net unrealized investment gains [Roll Forward]
Changes in net unrealized investment gains	513	(915)	(893)
Changes in Net Unrealized Gains on Investment Securities [Member] | Equity securities [Member]
Changes in net unrealized investment gains [Roll Forward]
Changes in net unrealized investment gains	(215)	51	(143)
Changes in Net Unrealized Gains on Investment Securities [Member] | Other investments [Member]
Changes in net unrealized investment gains [Roll Forward]
Changes in net unrealized investment gains	$ 4	$ 2	$ 2