Investments in Fixed Maturity Securities (Investments in Fixed Maturity Securities) (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
investments with a carrying value held on deposit in custodial or trust accounts	$ 1,022,000,000
Total held to maturity, Amortized Cost	88,526,000	$ 96,267,000
Total held to maturity, Gross Unrealized Gains	14,146,000	18,453,000
Total held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	102,672,000	114,720,000
Total held to maturity, Carrying Value	88,526,000	96,267,000
Total available for sale, Amortized Cost	12,869,809,000	12,012,041,000
Total available for sale, Gross Unrealized Gains	326,916,000	427,492,000
Total available for sale, Gross Unrealized Losses	(94,583,000)	(91,406,000)
Total available for sale, Fair Value	13,102,142,000	12,348,127,000
Total available for sale, Carrying Value	13,102,142,000	12,348,127,000
Total investment in fixed maturity securities, Amortized Cost	12,958,335,000	12,108,308,000
Total investments in fixed maturity securities, Gross Unrealized Gains	341,062,000	445,945,000
Total investments in fixed maturity securities, Gross Unrealized Losses	(94,583,000)	(91,406,000)
Total investments in fixed maturity securities, Fair Value	13,204,814,000	12,462,847,000
Total investments in fixed maturity securities, carrying value	13,190,668,000	12,444,394,000
Foreign Government Debt Securities [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	858,773,000	799,839,000
Total available for sale, Gross Unrealized Gains	46,794,000	50,310,000
Total available for sale, Gross Unrealized Losses	(2,762,000)	(12,689,000)
Total available for sale, Fair Value	902,805,000	837,460,000
Total available for sale, Carrying Value	902,805,000	837,460,000
Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	155,540,000	64,975,000
Total available for sale, Gross Unrealized Gains	304,000	875,000
Total available for sale, Gross Unrealized Losses	(2,981,000)	(128,000)
Total available for sale, Fair Value	152,863,000	65,722,000
Total available for sale, Carrying Value	152,863,000	65,722,000
U.S. government and government agency
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	496,187,000	645,092,000
Total available for sale, Gross Unrealized Gains	20,208,000	27,660,000
Total available for sale, Gross Unrealized Losses	(2,593,000)	(2,333,000)
Total available for sale, Fair Value	513,802,000	670,419,000
Total available for sale, Carrying Value	513,802,000	670,419,000
Held to maturity:
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	4,443,383,000	4,265,106,000
Total available for sale, Gross Unrealized Gains	116,558,000	201,342,000
Total available for sale, Gross Unrealized Losses	(40,438,000)	(6,269,000)
Total available for sale, Fair Value	4,519,503,000	4,460,179,000
Total available for sale, Carrying Value	4,519,503,000	4,460,179,000
State and municipal
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	818,691	1,269,491
Total available for sale, Amortized Cost	1,034,301,000	1,126,382,000
Total available for sale, Gross Unrealized Gains	15,431,000	18,935,000
Total available for sale, Gross Unrealized Losses	(12,950,000)	(11,180,000)
Total available for sale, Fair Value	1,036,782,000	1,134,137,000
Total available for sale, Carrying Value	1,036,782,000	1,134,137,000
Residential mortgage-backed
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	3,967,795,000	3,403,953,000
Total available for sale, Gross Unrealized Gains	121,650,000	115,478,000
Total available for sale, Gross Unrealized Losses	(20,918,000)	(44,393,000)
Total available for sale, Fair Value	4,068,527,000	3,475,038,000
Total available for sale, Carrying Value	4,068,527,000	3,475,038,000
Debt Securities
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	2,791,211,000	2,510,816,000
Total available for sale, Gross Unrealized Gains	58,559,000	102,909,000
Total available for sale, Gross Unrealized Losses	(26,315,000)	(3,737,000)
Total available for sale, Fair Value	2,823,455,000	2,609,988,000
Total available for sale, Carrying Value	2,823,455,000	2,609,988,000
Debt, State Government [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	524,682,000	583,456,000
Total available for sale, Gross Unrealized Gains	16,964,000	28,068,000
Total available for sale, Gross Unrealized Losses	(5,139,000)	(2,070,000)
Total available for sale, Fair Value	536,507,000	609,454,000
Total available for sale, Carrying Value	536,507,000	609,454,000
Revenue Subject to Refund [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	356,535,000	439,772,000
Total available for sale, Gross Unrealized Gains	19,181,000	32,056,000
Total available for sale, Gross Unrealized Losses	(165,000)	(31,000)
Total available for sale, Fair Value	375,551,000	471,797,000
Total available for sale, Carrying Value	375,551,000	471,797,000
Corporation [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	410,933,000	388,904,000
Total available for sale, Gross Unrealized Gains	6,172,000	14,039,000
Total available for sale, Gross Unrealized Losses	(6,452,000)	(402,000)
Total available for sale, Fair Value	410,653,000	402,541,000
Total available for sale, Carrying Value	410,653,000	402,541,000
Collateralized Mortgage Backed Securities [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	1,189,841,000	1,191,357,000
Total available for sale, Gross Unrealized Gains	15,735,000	19,810,000
Total available for sale, Gross Unrealized Losses	(15,931,000)	(11,308,000)
Total available for sale, Fair Value	1,189,645,000	1,199,859,000
Total available for sale, Carrying Value	1,189,645,000	1,199,859,000
Asset-backed Securities [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	1,913,830,000	1,706,694,000
Total available for sale, Gross Unrealized Gains	5,971,000	12,892,000
Total available for sale, Gross Unrealized Losses	(11,941,000)	(14,414,000)
Total available for sale, Fair Value	1,907,860,000	1,705,172,000
Total available for sale, Carrying Value	1,907,860,000	1,705,172,000
Industrial Property [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	2,315,567,000	1,976,393,000
Total available for sale, Gross Unrealized Gains	71,007,000	75,168,000
Total available for sale, Gross Unrealized Losses	(7,174,000)	(30,027,000)
Total available for sale, Fair Value	2,379,400,000	2,021,534,000
Total available for sale, Carrying Value	2,379,400,000	2,021,534,000
Domestic Corporate Debt Securities [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	1,369,001,000	1,153,096,000
Total available for sale, Gross Unrealized Gains	39,543,000	31,744,000
Total available for sale, Gross Unrealized Losses	(11,270,000)	(11,819,000)
Total available for sale, Fair Value	1,397,274,000	1,173,021,000
Total available for sale, Carrying Value	1,397,274,000	1,173,021,000
Public Utility, Bonds [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	229,154,000	192,857,000
Total available for sale, Gross Unrealized Gains	10,801,000	8,321,000
Total available for sale, Gross Unrealized Losses	(2,411,000)	(2,527,000)
Total available for sale, Fair Value	237,544,000	198,651,000
Total available for sale, Carrying Value	237,544,000	198,651,000
Other Security Investments [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	54,073,000	81,607,000
Total available for sale, Gross Unrealized Gains	299,000	245,000
Total available for sale, Gross Unrealized Losses	(63,000)	(20,000)
Total available for sale, Fair Value	54,309,000	81,832,000
Total available for sale, Carrying Value	54,309,000	81,832,000
Debt, Local Government [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	360,022,000	342,158,000
Total available for sale, Gross Unrealized Gains	15,682,000	24,270,000
Total available for sale, Gross Unrealized Losses	(2,367,000)	(29,000)
Total available for sale, Fair Value	373,337,000	366,399,000
Total available for sale, Carrying Value	373,337,000	366,399,000
523991 Trust, Fiduciary, and Custody Activities [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
investments with a carrying value held on deposit in custodial or trust accounts	1,261,000,000
State and municipal
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total held to maturity, Amortized Cost	15,944,000	19,138,000
Total held to maturity, Gross Unrealized Gains	1,693,000	2,207,000
Total held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	17,637,000	21,345,000
Total held to maturity, Carrying Value	15,944,000	19,138,000
Held to maturity:
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total held to maturity, Amortized Cost	72,582,000	77,129,000
Total held to maturity, Gross Unrealized Gains	12,453,000	16,246,000
Total held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	85,035,000	93,375,000
Total held to maturity, Carrying Value	$ 72,582,000	$ 77,129,000