Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reconciliation of the change in the amount of unrecognized tax benefits
Balance – beginning of year	$ 0	$ 0	$ 0	$ 25
Increase for tax positions taken in a prior year		4	0	1
Decrease for tax positions taken in a prior year		0	0	0
Increase for tax positions taken in the current year		3	0	0
Decrease for tax positions taken in the current year		0	0	0
Decrease for settlements		0	0	(26)
Reductions due to lapse of statute of limitations		0	0	0
Balance – end of year		7	0	0
Net operating loss carryforward		58
Deferred assets
Unearned premium reserves		796	763
Pension		236	254
Discount on loss reserves		203	210
Difference in tax bases of invested assets		202	64
Accrued compensation		189	206
Other postretirement benefits		76	138
Other assets		137	138
Total deferred assets		1,839	1,773
Deferred liabilities
DAC		(1,157)	(1,076)
Unrealized net capital gains		(303)	(994)
Life and annuity reserves		(260)	(192)
Other liabilities		(209)	(226)
Total deferred liabilities		(1,929)	(2,488)
Net deferred liability		(90)	(715)
Components of income tax expense (benefit)
Current		1,033	1,123	869
Deferred		78	263	247
Total income tax expense		1,111	1,386	1,116
Income tax paid		1,070	1,070	$ 500
Current income tax receivable		$ 59
Current income tax payable			$ 158
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation
Statutory federal income tax rate (as a percent)		35.00%	35.00%	35.00%
Tax-exempt income (as a percent)		(1.00%)	(0.90%)	(1.80%)
Tax credits (as a percent)		(0.90%)	(0.70%)	(2.20%)
Sale of subsidiary (as a percent)		0.00%	(0.90%)	2.00%
Other (as a percent)		0.80%	0.20%	(0.10%)
Effective income tax rate (as a percent)		33.90%	32.70%	32.90%
Accounting Standards Update 2014-01
Components of income tax expense (benefit)
Total income tax expense	$ 45