Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	$ 78,547,000	$ 78,635,000
Held to maturity, Gross Unrealized Gains	14,172,000	12,808,000
Held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	92,719,000	91,443,000
Total held to maturity, Carrying Value	78,547,000	78,635,000
Total available for sale, Amortized Cost	13,400,996,000	13,596,057,000
Total available for sale, Gross Unrealized Gains	329,002,000	147,683,000
Total available for sale, Gross Unrealized Losses	(75,745,000)	(215,563,000)
Total available for sale, Fair Value	13,654,253,000	13,528,177,000
Total available for sale, Carrying Value	13,654,253,000	13,528,177,000
Total investments in fixed maturity securities, Amortized Value	13,479,543,000	13,674,692,000
Total investments in fixed maturity securities, Gross Unrealized Gains	343,174,000	160,491,000
Total investments in fixed maturity securities, Gross Unrealized Losses	(75,745,000)	(215,563,000)
Total investments in fixed maturity securities, Fair Value	13,746,972,000	13,619,620,000
Total investments in fixed maturity securities, Carrying Value	13,732,800,000	13,606,812,000
State and municipal
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	69,167,000	67,891,000
Held to maturity, Gross Unrealized Gains	13,089,000	11,549,000
Held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	82,256,000	79,440,000
Total held to maturity, Carrying Value	69,167,000	67,891,000
Total available for sale, Amortized Cost	3,607,465,000	3,815,515,000
Total available for sale, Gross Unrealized Gains	144,279,000	81,695,000
Total available for sale, Gross Unrealized Losses	(3,921,000)	(24,612,000)
Total available for sale, Fair Value	3,747,823,000	3,872,598,000
Total available for sale, Carrying Value	3,747,823,000	3,872,598,000
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	9,380,000	10,744,000
Held to maturity, Gross Unrealized Gains	1,083,000	1,259,000
Held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	10,463,000	12,003,000
Total held to maturity, Carrying Value	9,380,000	10,744,000
Total available for sale, Amortized Cost	1,242,880,000	1,264,376,000
Total available for sale, Gross Unrealized Gains	28,521,000	7,729,000
Total available for sale, Gross Unrealized Losses	(5,316,000)	(20,225,000)
Total available for sale, Fair Value	1,266,085,000	1,251,880,000
Total available for sale, Carrying Value	1,266,085,000	1,251,880,000
Gross unrealized gains (losses) related to securities	(42,762)	(55,090)
U.S. government and government agency
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	760,247,000	697,931,000
Total available for sale, Gross Unrealized Gains	18,241,000	9,219,000
Total available for sale, Gross Unrealized Losses	(1,418,000)	(4,910,000)
Total available for sale, Fair Value	777,070,000	702,240,000
Total available for sale, Carrying Value	777,070,000	702,240,000
Special revenue
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	2,322,750,000	2,396,089,000
Total available for sale, Gross Unrealized Gains	63,204,000	30,507,000
Total available for sale, Gross Unrealized Losses	(2,674,000)	(19,790,000)
Total available for sale, Fair Value	2,383,280,000	2,406,806,000
Total available for sale, Carrying Value	2,383,280,000	2,406,806,000
State general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	320,338,000	335,626,000
Total available for sale, Gross Unrealized Gains	22,108,000	11,951,000
Total available for sale, Gross Unrealized Losses	(43,000)	(1,103,000)
Total available for sale, Fair Value	342,403,000	346,474,000
Total available for sale, Carrying Value	342,403,000	346,474,000
Pre-refunded
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	298,825,000	408,141,000
Total available for sale, Gross Unrealized Gains	19,876,000	16,568,000
Total available for sale, Gross Unrealized Losses	(122,000)	(30,000)
Total available for sale, Fair Value	318,579,000	424,679,000
Total available for sale, Carrying Value	318,579,000	424,679,000
Corporate backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	238,319,000	272,440,000
Total available for sale, Gross Unrealized Gains	7,065,000	4,319,000
Total available for sale, Gross Unrealized Losses	(639,000)	(2,350,000)
Total available for sale, Fair Value	244,745,000	274,409,000
Total available for sale, Carrying Value	244,745,000	274,409,000
Local general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	427,233,000	403,219,000
Total available for sale, Gross Unrealized Gains	32,026,000	18,350,000
Total available for sale, Gross Unrealized Losses	(443,000)	(1,339,000)
Total available for sale, Fair Value	458,816,000	420,230,000
Total available for sale, Carrying Value	458,816,000	420,230,000
Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	303,332,000	345,070,000
Total available for sale, Gross Unrealized Gains	6,829,000	1,304,000
Total available for sale, Gross Unrealized Losses	(380,000)	(3,708,000)
Total available for sale, Fair Value	309,781,000	342,666,000
Total available for sale, Carrying Value	309,781,000	342,666,000
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,546,212,000	1,609,446,000
Total available for sale, Gross Unrealized Gains	35,350,000	9,033,000
Total available for sale, Gross Unrealized Losses	(5,696,000)	(23,933,000)
Total available for sale, Fair Value	1,575,866,000	1,594,546,000
Total available for sale, Carrying Value	1,575,866,000	1,594,546,000
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	2,719,618,000	2,462,303,000
Total available for sale, Gross Unrealized Gains	9,874,000	10,131,000
Total available for sale, Gross Unrealized Losses	(27,707,000)	(33,687,000)
Total available for sale, Fair Value	2,701,785,000	2,438,747,000
Total available for sale, Carrying Value	2,701,785,000	2,438,747,000
Industrial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	2,178,339,000	2,295,778,000
Total available for sale, Gross Unrealized Gains	59,623,000	15,355,000
Total available for sale, Gross Unrealized Losses	(7,755,000)	(53,312,000)
Total available for sale, Fair Value	2,230,207,000	2,257,821,000
Total available for sale, Carrying Value	2,230,207,000	2,257,821,000
Financial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,431,612,000	1,502,427,000
Total available for sale, Gross Unrealized Gains	27,484,000	7,178,000
Total available for sale, Gross Unrealized Losses	(5,373,000)	(45,683,000)
Total available for sale, Fair Value	1,453,723,000	1,463,922,000
Total available for sale, Carrying Value	1,453,723,000	1,463,922,000
Utilities
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	312,200,000	330,326,000
Total available for sale, Gross Unrealized Gains	12,548,000	2,997,000
Total available for sale, Gross Unrealized Losses	(426,000)	(4,148,000)
Total available for sale, Fair Value	324,322,000	329,175,000
Total available for sale, Carrying Value	324,322,000	329,175,000
Other
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	27,027,000	60,238,000
Total available for sale, Gross Unrealized Gains	1,096,000	322,000
Total available for sale, Gross Unrealized Losses	(90,000)	(167,000)
Total available for sale, Fair Value	28,033,000	60,393,000
Total available for sale, Carrying Value	28,033,000	60,393,000
Corporate
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,949,178,000	4,188,769,000
Total available for sale, Gross Unrealized Gains	100,751,000	25,852,000
Total available for sale, Gross Unrealized Losses	(13,644,000)	(103,310,000)
Total available for sale, Fair Value	4,036,285,000	4,111,311,000
Total available for sale, Carrying Value	4,036,285,000	4,111,311,000
Foreign government
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	818,276,000	822,093,000
Total available for sale, Gross Unrealized Gains	20,507,000	11,753,000
Total available for sale, Gross Unrealized Losses	(23,359,000)	(25,111,000)
Total available for sale, Fair Value	815,424,000	808,735,000
Total available for sale, Carrying Value	$ 815,424,000	$ 808,735,000