Investments in Fixed Maturity Securities (Investments in Fixed Maturity Securities) (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total held to maturity, Amortized Cost	$ 101,177,000		$ 101,320,000
Total held to maturity, Gross Unrealized Gains	20,646,000		14,900,000
Total held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	121,823,000		116,220,000
Total held to maturity, Carrying Value	101,177,000		101,320,000
Total available for sale, Amortized Cost	12,122,087,000		11,190,311,000
Total available for sale, Gross Unrealized Gains	541,420,000		436,347,000
Total available for sale, Gross Unrealized Losses	(59,524,000)		(111,134,000)
Total available for sale, Fair Value	12,603,983,000		11,515,524,000
Total available for sale, Carrying Value	12,603,983,000		11,515,524,000
Total investment in fixed maturity securities, Amortized Cost	12,223,264,000		11,291,631,000
Total investments in fixed maturity securities, Gross Unrealized Gains	562,066,000		451,247,000
Total investments in fixed maturity securities, Gross Unrealized Losses	(59,524,000)		(111,134,000)
Total investments in fixed maturity securities, Fair Value	12,725,806,000		11,631,744,000
Total investments in fixed maturity securities, carrying value	12,705,160,000		11,616,844,000
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	773,192,000		858,319,000
Total available for sale, Gross Unrealized Gains	33,353,000		34,522,000
Total available for sale, Gross Unrealized Losses	(3,157,000)		(7,982,000)
Total available for sale, Fair Value	803,388,000		884,859,000
Total available for sale, Carrying Value	803,388,000		884,859,000
State and municipal
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	4,137,866,000		4,085,791,000
Total available for sale, Gross Unrealized Gains	229,150,000		162,330,000
Total available for sale, Gross Unrealized Losses	(4,020,000)		(29,837,000)
Total available for sale, Fair Value	4,362,996,000		4,218,284,000
Total available for sale, Carrying Value	4,362,996,000		4,218,284,000
Residential mortgage-backed securities
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	1,201,924,000		1,248,693,000	[1]
Total available for sale, Gross Unrealized Gains	27,124,000		25,895,000	[1]
Total available for sale, Gross Unrealized Losses	(9,449,000)		(25,941,000)	[1]
Total available for sale, Fair Value	1,219,599,000	[1]	1,248,647,000	[1]
Total available for sale, Carrying Value	1,219,599,000		1,248,647,000	[1]
Non-credit portion of other-than-temporary impairments ("OTTI")	1,095,671		1,961,247
Commercial
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	74,479,000		76,454,000
Total available for sale, Gross Unrealized Gains	1,610,000		5,670,000
Total available for sale, Gross Unrealized Losses	(52,000)		(988,000)
Total available for sale, Fair Value	76,037,000		81,136,000
Total available for sale, Carrying Value	76,037,000		81,136,000
Corporate
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	5,036,958,000		4,076,585,000
Total available for sale, Gross Unrealized Gains	187,960,000		156,256,000
Total available for sale, Gross Unrealized Losses	(24,781,000)		(30,100,000)
Total available for sale, Fair Value	5,200,137,000		4,202,741,000
Total available for sale, Carrying Value	5,200,137,000		4,202,741,000
Foreign government
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	897,668,000		844,469,000
Total available for sale, Gross Unrealized Gains	62,223,000		51,674,000
Total available for sale, Gross Unrealized Losses	(18,065,000)		(16,286,000)
Total available for sale, Fair Value	941,826,000		879,857,000
Total available for sale, Carrying Value	941,826,000		879,857,000
State and municipal
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total held to maturity, Amortized Cost	72,901,000		68,929,000
Total held to maturity, Gross Unrealized Gains	17,501,000		11,172,000
Total held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	90,402,000		80,101,000
Total held to maturity, Carrying Value	72,901,000		68,929,000
Residential mortgage-backed securities
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total held to maturity, Amortized Cost	23,278,000		27,393,000
Total held to maturity, Gross Unrealized Gains	2,854,000		3,311,000
Total held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	26,132,000		30,704,000
Total held to maturity, Carrying Value	23,278,000		27,393,000
Corporate
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total held to maturity, Amortized Cost	4,998,000		4,998,000
Total held to maturity, Gross Unrealized Gains	291,000		417,000
Total held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	5,289,000		5,415,000
Total held to maturity, Carrying Value	$ 4,998,000		$ 4,998,000

[1] Gross unrealized losses for mortgage-backed securities include $1,095,671 and $1,961,247, as of December 31, 2014 and 2013, respectively, related to the non-credit portion of OTTI recognized in other comprehensive income.