Investments (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2016	Dec. 31, 2015
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost/Cost		$ 6,791,301	$ 6,785,626
Unrealized Gains		489,801	395,237
Unrealized Losses		64,600	89,523
Fair Value		7,216,502	7,091,340
OTTI in AOCI	[1]	(1,460)	(2,758)
Amortized Cost/Cost, Equity securities		96,305	95,722
Fair Value, Equity securities		104,044	99,797
Mortgage-backed securities [Member]
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost/Cost	[2]	441,038	461,862
Unrealized Gains	[2]	54,633	44,413
Unrealized Losses	[2]	471	1,861
Fair Value	[2]	495,200	504,414
OTTI in AOCI	[1],[2]	0	0
Other, including U.S. Treasury Securities [Member]
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost/Cost	[2]	508,042	532,373
Unrealized Gains	[2]	32,078	21,153
Unrealized Losses	[2]	0	7,415
Fair Value	[2]	540,120	546,111
OTTI in AOCI	[1],[2]	0	0
Municipal bonds [Member]
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost/Cost		1,536,111	1,553,603
Unrealized Gains		198,416	165,680
Unrealized Losses		7,199	10,340
Fair Value		1,727,328	1,708,943
OTTI in AOCI	[1]	(2,724)	(4,140)
Foreign government bonds [Member]
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost/Cost		67,422	67,441
Unrealized Gains		6,665	6,288
Unrealized Losses		0	112
Fair Value		74,087	73,617
OTTI in AOCI	[1]	0	0
Corporate bonds [Member]
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost/Cost		2,684,520	2,687,376
Unrealized Gains		177,754	140,873
Unrealized Losses		28,343	48,834
Fair Value		2,833,931	2,779,415
OTTI in AOCI	[1]	0	0
Other mortgage-backed securities [Member]
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost/Cost		1,554,168	1,482,971
Unrealized Gains		20,255	16,830
Unrealized Losses		28,587	20,961
Fair Value		1,545,836	1,478,840
OTTI in AOCI	[1]	1,264	1,382
Equity securities [Member]
Unrealized gains and losses on fixed maturities and equity securities
OTTI in AOCI	[1],[3]	0	0
Amortized Cost/Cost, Equity securities	[3]	96,305	95,722
Unrealized Gains, Equity securities	[3]	10,837	8,405
Unrealized Losses, Equity securities	[3]	3,098	4,330
Fair Value, Equity securities	[3]	$ 104,044	$ 99,797

[1] Related to securities for which an unrealized loss was bifurcated to distinguish the credit-related portion and the portion driven by other market factors. Represents the amount of other-than-temporary impairment losses in AOCI which was not included in earnings; amounts also include unrealized gains/(losses) on such impaired securities relating to changes in the fair value of those securities subsequent to the impairment measurement date.

[2] Fair value includes securities issued by Federal National Mortgage Association (“FNMA”) of $234,668 and $231,294; Federal Home Loan Mortgage Corporation (“FHLMC”) of $342,877 and $363,957; and Government National Mortgage Association (“GNMA”) of $126,252 and $130,940 as of March 31, 2016 and December 31, 2015, respectively.

[3] Includes nonredeemable (perpetual) preferred stocks, common stocks and closed-end funds.