Significant Accounting Policies Significant Accounting Policies - Summary of Impact of the Retrospectively Adjusted Balances on the Consolidated Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014		Dec. 31, 2013
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net investment income	$ 194,319	$ 193,213	$ 194,823	$ 191,596	$ 190,717	$ 189,588	$ 189,930	$ 188,051	$ 773,951	$ 758,286	[1]	$ 734,650	[1]
Total revenue	933,860	947,154	950,611	934,440	917,175	890,834	898,343	913,743	3,766,065	3,620,095	[1]	3,494,253	[1]
Income before income taxes	175,978	199,009	196,723	194,477	199,009	184,709	191,922	209,037	766,187	784,677	[1]	755,315	[1]
Income taxes									(249,894)	(256,603)	[1]	(248,110)	[1]
Net income	$ 132,956	$ 145,386	$ 127,110	$ 121,648	$ 146,748	$ 132,412	$ 130,923	$ 132,856	$ 527,100	542,939	[1],[2]	528,472	[1],[2]
As previously reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net investment income										729,207		709,743
Total revenue										3,591,016		3,469,346
Income before income taxes										755,598		730,408
Income taxes										(227,524)		(223,203)
Net income										542,939		528,472
Adjustments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net investment income										29,079		24,907
Total revenue										29,079		24,907
Income before income taxes										29,079		24,907
Income taxes										(29,079)		(24,907)
Net income										$ 0		$ 0

[1] Certain balances were adjusted to give effect to discontinued operations and the adoption of new accounting guidance as described in Note 1- Significant Accounting Policies under the caption "Low-Income Housing Tax Credit Interests."

[2] Certain balances were adjusted to give effect to discontinued operations as described in Note 1—Significant Accounting Policies.