Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Total held to maturity, Amortized Cost	$ 79,396,000	$ 88,526,000
Held to Maturity Securities Accumulated Unrecognized Holding Gain	16,664,000	14,146,000
Held to Maturity Securities Accumulated Unrecognized Holding Loss	0	0
Total held to maturity, Fair Value	96,060,000	102,672,000
Total held to maturity, Carrying Value	79,396,000	88,526,000
Total available for sale, Amortized Cost	13,479,715,000	12,869,809,000
Total available for sale, Gross Unrealized Gains	355,256,000	326,916,000
Total available for sale, Gross Unrealized Losses	(40,677,000)	(94,583,000)
Total available for sale, Fair Value	13,794,294,000	13,102,142,000
Total available for sale, Carrying Value	13,794,294,000	13,102,142,000
Total investments in fixed maturity securities, Amortized Value	13,559,111,000	12,958,335,000
Total investments in fixed maturity securities, Gross Unrealized Gains	371,920,000	341,062,000
Total investments in fixed maturity securities, Gross Unrealized Losses	(40,677,000)	(94,583,000)
Total investments in fixed maturity securities, Fair Value	13,890,354,000	13,204,814,000
Total investments in fixed maturity securities, Carrying Value	$ 13,873,690,000	13,190,668,000
Minimum ownership of Common stockholders equity	10.00%
U.S. government and government agency
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	$ 402,901,000	496,187,000
Total available for sale, Gross Unrealized Gains	10,828,000	20,208,000
Total available for sale, Gross Unrealized Losses	(2,124,000)	(2,593,000)
Total available for sale, Fair Value	411,605,000	513,802,000
Total available for sale, Carrying Value	411,605,000	513,802,000
Special revenue
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	2,730,653,000	2,791,211,000
Total available for sale, Gross Unrealized Gains	80,144,000	58,559,000
Total available for sale, Gross Unrealized Losses	(5,469,000)	(26,315,000)
Total available for sale, Fair Value	2,805,328,000	2,823,455,000
Total available for sale, Carrying Value	2,805,328,000	2,823,455,000
State general obligation
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	481,070,000	524,682,000
Total available for sale, Gross Unrealized Gains	21,377,000	16,964,000
Total available for sale, Gross Unrealized Losses	(565,000)	(5,139,000)
Total available for sale, Fair Value	501,882,000	536,507,000
Total available for sale, Carrying Value	501,882,000	536,507,000
Pre-refunded
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	282,488,000	356,535,000
Total available for sale, Gross Unrealized Gains	21,052,000	19,181,000
Total available for sale, Gross Unrealized Losses	(173,000)	(165,000)
Total available for sale, Fair Value	303,367,000	375,551,000
Total available for sale, Carrying Value	303,367,000	375,551,000
Corporate backed
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	380,351,000	410,933,000
Total available for sale, Gross Unrealized Gains	11,775,000	6,172,000
Total available for sale, Gross Unrealized Losses	(499,000)	(6,452,000)
Total available for sale, Fair Value	391,627,000	410,653,000
Total available for sale, Carrying Value	391,627,000	410,653,000
Local general obligation
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	384,930,000	360,022,000
Total available for sale, Gross Unrealized Gains	26,566,000	15,682,000
Total available for sale, Gross Unrealized Losses	(531,000)	(2,367,000)
Total available for sale, Fair Value	410,965,000	373,337,000
Total available for sale, Carrying Value	410,965,000	373,337,000
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	4,259,492,000	4,443,383,000
Total available for sale, Gross Unrealized Gains	160,914,000	116,558,000
Total available for sale, Gross Unrealized Losses	(7,237,000)	(40,438,000)
Total available for sale, Fair Value	4,413,169,000	4,519,503,000
Total available for sale, Carrying Value	4,413,169,000	4,519,503,000
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	1,316,092,000	1,189,841,000
Total available for sale, Gross Unrealized Gains	14,337,000	15,735,000
Total available for sale, Gross Unrealized Losses	(10,310,000)	(15,931,000)
Total available for sale, Fair Value	1,320,119,000	1,189,645,000
Total available for sale, Carrying Value	1,320,119,000	1,189,645,000
Residential
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	1,064,705,000	1,034,301,000
Total available for sale, Gross Unrealized Gains	12,696,000	15,431,000
Total available for sale, Gross Unrealized Losses	(8,860,000)	(12,950,000)
Total available for sale, Fair Value	1,068,541,000	1,036,782,000
Total available for sale, Carrying Value	1,068,541,000	1,036,782,000
Gross unrealized gains (losses) related to securities	85,907	(818,691)
Commercial
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	251,387,000	155,540,000
Total available for sale, Gross Unrealized Gains	1,641,000	304,000
Total available for sale, Gross Unrealized Losses	(1,450,000)	(2,981,000)
Total available for sale, Fair Value	251,578,000	152,863,000
Total available for sale, Carrying Value	251,578,000	152,863,000
Asset-backed
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	2,389,187,000	1,913,830,000
Total available for sale, Gross Unrealized Gains	9,836,000	5,971,000
Total available for sale, Gross Unrealized Losses	(10,405,000)	(11,941,000)
Total available for sale, Fair Value	2,388,618,000	1,907,860,000
Total available for sale, Carrying Value	2,388,618,000	1,907,860,000
Corporate
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	4,202,435,000	3,967,795,000
Total available for sale, Gross Unrealized Gains	126,452,000	121,650,000
Total available for sale, Gross Unrealized Losses	(8,513,000)	(20,918,000)
Total available for sale, Fair Value	4,320,374,000	4,068,527,000
Total available for sale, Carrying Value	4,320,374,000	4,068,527,000
Industrial
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	2,569,673,000	2,315,567,000
Total available for sale, Gross Unrealized Gains	70,054,000	71,007,000
Total available for sale, Gross Unrealized Losses	(2,842,000)	(7,174,000)
Total available for sale, Fair Value	2,636,885,000	2,379,400,000
Total available for sale, Carrying Value	2,636,885,000	2,379,400,000
Financial
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	1,335,101,000	1,369,001,000
Total available for sale, Gross Unrealized Gains	43,636,000	39,543,000
Total available for sale, Gross Unrealized Losses	(4,716,000)	(11,270,000)
Total available for sale, Fair Value	1,374,021,000	1,397,274,000
Total available for sale, Carrying Value	1,374,021,000	1,397,274,000
Utilities
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	255,478,000	229,154,000
Total available for sale, Gross Unrealized Gains	12,760,000	10,801,000
Total available for sale, Gross Unrealized Losses	(907,000)	(2,411,000)
Total available for sale, Fair Value	267,331,000	237,544,000
Total available for sale, Carrying Value	267,331,000	237,544,000
Other
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	42,183,000	54,073,000
Total available for sale, Gross Unrealized Gains	2,000	299,000
Total available for sale, Gross Unrealized Losses	(48,000)	(63,000)
Total available for sale, Fair Value	42,137,000	54,309,000
Total available for sale, Carrying Value	42,137,000	54,309,000
Foreign
Schedule of Available-for-sale Securities [Line Items]
Total available for sale, Amortized Cost	909,608,000	858,773,000
Total available for sale, Gross Unrealized Gains	32,889,000	46,794,000
Total available for sale, Gross Unrealized Losses	(2,088,000)	(2,762,000)
Total available for sale, Fair Value	940,409,000	902,805,000
Total available for sale, Carrying Value	940,409,000	902,805,000
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Total held to maturity, Amortized Cost	65,372,000	72,582,000
Held to Maturity Securities Accumulated Unrecognized Holding Gain	15,212,000	12,453,000
Held to Maturity Securities Accumulated Unrecognized Holding Loss	0	0
Total held to maturity, Fair Value	80,584,000	85,035,000
Total held to maturity, Carrying Value	65,372,000	72,582,000
Residential
Schedule of Available-for-sale Securities [Line Items]
Total held to maturity, Amortized Cost	14,024,000	15,944,000
Held to Maturity Securities Accumulated Unrecognized Holding Gain	1,452,000	1,693,000
Held to Maturity Securities Accumulated Unrecognized Holding Loss	0	0
Total held to maturity, Fair Value	15,476,000	17,637,000
Total held to maturity, Carrying Value	$ 14,024,000	$ 15,944,000