Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Total held to maturity, Amortized Cost	$ 79,231,000	$ 79,332,000
Held to Maturity Securities Accumulated Unrecognized Holding Gain	13,173,000	15,726,000
Held to Maturity Securities Accumulated Unrecognized Holding Loss	0	0
Total held to maturity, Fair Value	92,404,000	95,058,000
Total held to maturity, Carrying Value	79,231,000	79,332,000
Total available for sale, Amortized Cost	13,194,278,000	13,242,580,000
Total available for sale, Gross Unrealized Gains	199,669,000	289,567,000
Total available for sale, Gross Unrealized Losses	(130,046,000)	(60,229,000)
Total available for sale, Fair Value	13,263,901,000	13,471,918,000
Total available for sale, Carrying Value	13,263,901,000	13,471,918,000
Total investments in fixed maturity securities, Amortized Value	13,273,509,000	13,321,912,000
Total investments in fixed maturity securities, Gross Unrealized Gains	212,842,000	305,293,000
Total investments in fixed maturity securities, Gross Unrealized Losses	(130,046,000)	(60,229,000)
Total investments in fixed maturity securities, Fair Value	13,356,305,000	13,566,976,000
Total investments in fixed maturity securities, Carrying Value	13,343,132,000	13,551,250,000
U.S. government and government agency
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	479,330,000	372,748,000
Total available for sale, Gross Unrealized Gains	6,692,000	8,824,000
Total available for sale, Gross Unrealized Losses	(6,113,000)	(3,832,000)
Total available for sale, Fair Value	479,909,000	377,740,000
Total available for sale, Carrying Value	479,909,000	377,740,000
Special revenue
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	2,644,248,000	2,663,245,000
Total available for sale, Gross Unrealized Gains	36,059,000	53,512,000
Total available for sale, Gross Unrealized Losses	(22,675,000)	(10,027,000)
Total available for sale, Fair Value	2,657,632,000	2,706,730,000
Total available for sale, Carrying Value	2,657,632,000	2,706,730,000
State general obligation
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	404,710,000	439,358,000
Total available for sale, Gross Unrealized Gains	12,113,000	16,087,000
Total available for sale, Gross Unrealized Losses	(2,610,000)	(711,000)
Total available for sale, Fair Value	414,213,000	454,734,000
Total available for sale, Carrying Value	414,213,000	454,734,000
Pre-refunded
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	443,852,000	436,241,000
Total available for sale, Gross Unrealized Gains	18,131,000	22,701,000
Total available for sale, Gross Unrealized Losses	(87,000)	(9,000)
Total available for sale, Fair Value	461,896,000	458,933,000
Total available for sale, Carrying Value	461,896,000	458,933,000
Corporate backed
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	368,073,000	375,268,000
Total available for sale, Gross Unrealized Gains	7,415,000	10,059,000
Total available for sale, Gross Unrealized Losses	(1,610,000)	(860,000)
Total available for sale, Fair Value	373,878,000	384,467,000
Total available for sale, Carrying Value	373,878,000	384,467,000
Local general obligation
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	389,887,000	417,955,000
Total available for sale, Gross Unrealized Gains	17,227,000	23,242,000
Total available for sale, Gross Unrealized Losses	(3,209,000)	(967,000)
Total available for sale, Fair Value	403,905,000	440,230,000
Total available for sale, Carrying Value	403,905,000	440,230,000
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	4,250,770,000	4,332,067,000
Total available for sale, Gross Unrealized Gains	90,945,000	125,601,000
Total available for sale, Gross Unrealized Losses	(30,191,000)	(12,574,000)
Total available for sale, Fair Value	4,311,524,000	4,445,094,000
Total available for sale, Carrying Value	4,311,524,000	4,445,094,000
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	1,406,497,000	1,305,281,000
Total available for sale, Gross Unrealized Gains	7,397,000	10,825,000
Total available for sale, Gross Unrealized Losses	(26,897,000)	(16,175,000)
Total available for sale, Fair Value	1,386,997,000	1,299,931,000
Total available for sale, Carrying Value	1,386,997,000	1,299,931,000
Residential
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	1,067,814,000	1,043,629,000
Total available for sale, Gross Unrealized Gains	6,233,000	9,304,000
Total available for sale, Gross Unrealized Losses	(22,334,000)	(13,547,000)
Total available for sale, Fair Value	1,051,713,000	1,039,386,000
Total available for sale, Carrying Value	1,051,713,000	1,039,386,000
Gross unrealized gains (losses) related to securities	89,704	76,467
Commercial
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	338,683,000	261,652,000
Total available for sale, Gross Unrealized Gains	1,164,000	1,521,000
Total available for sale, Gross Unrealized Losses	(4,563,000)	(2,628,000)
Total available for sale, Fair Value	335,284,000	260,545,000
Total available for sale, Carrying Value	335,284,000	260,545,000
Asset-backed
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	2,101,896,000	2,111,132,000
Total available for sale, Gross Unrealized Gains	11,435,000	11,024,000
Total available for sale, Gross Unrealized Losses	(11,640,000)	(10,612,000)
Total available for sale, Fair Value	2,101,691,000	2,111,544,000
Total available for sale, Carrying Value	2,101,691,000	2,111,544,000
Corporate
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	4,104,919,000	4,302,007,000
Total available for sale, Gross Unrealized Gains	57,409,000	101,275,000
Total available for sale, Gross Unrealized Losses	(51,627,000)	(14,170,000)
Total available for sale, Fair Value	4,110,701,000	4,389,112,000
Total available for sale, Carrying Value	4,110,701,000	4,389,112,000
Industrial
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	2,378,793,000	2,574,400,000
Total available for sale, Gross Unrealized Gains	27,145,000	52,210,000
Total available for sale, Gross Unrealized Losses	(31,817,000)	(7,718,000)
Total available for sale, Fair Value	2,374,121,000	2,618,892,000
Total available for sale, Carrying Value	2,374,121,000	2,618,892,000
Financial
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	1,411,529,000	1,402,161,000
Total available for sale, Gross Unrealized Gains	22,100,000	37,744,000
Total available for sale, Gross Unrealized Losses	(15,031,000)	(5,138,000)
Total available for sale, Fair Value	1,418,598,000	1,434,767,000
Total available for sale, Carrying Value	1,418,598,000	1,434,767,000
Utilities
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	270,915,000	284,886,000
Total available for sale, Gross Unrealized Gains	8,163,000	11,316,000
Total available for sale, Gross Unrealized Losses	(4,546,000)	(1,248,000)
Total available for sale, Fair Value	274,532,000	294,954,000
Total available for sale, Carrying Value	274,532,000	294,954,000
Other
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	43,682,000	40,560,000
Total available for sale, Gross Unrealized Gains	1,000	5,000
Total available for sale, Gross Unrealized Losses	(233,000)	(66,000)
Total available for sale, Fair Value	43,450,000	40,499,000
Total available for sale, Carrying Value	43,450,000	40,499,000
Foreign
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	850,866,000	819,345,000
Total available for sale, Gross Unrealized Gains	25,791,000	32,018,000
Total available for sale, Gross Unrealized Losses	(3,578,000)	(2,866,000)
Total available for sale, Fair Value	873,079,000	848,497,000
Total available for sale, Carrying Value	873,079,000	848,497,000
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Total held to maturity, Amortized Cost	66,459,000	65,882,000
Held to Maturity Securities Accumulated Unrecognized Holding Gain	12,185,000	14,499,000
Held to Maturity Securities Accumulated Unrecognized Holding Loss	0	0
Total held to maturity, Fair Value	78,644,000	80,381,000
Total held to maturity, Carrying Value	66,459,000	65,882,000
Residential
Schedule of Available-for-sale Securities [Line Items]
Total held to maturity, Amortized Cost	12,772,000	13,450,000
Held to Maturity Securities Accumulated Unrecognized Holding Gain	988,000	1,227,000
Held to Maturity Securities Accumulated Unrecognized Holding Loss	0	0
Total held to maturity, Fair Value	13,760,000	14,677,000
Total held to maturity, Carrying Value	$ 12,772,000	$ 13,450,000