Investments (Details) (USD $)	Mar. 31, 2014		Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 4,806,454,000		$ 4,951,451,000
Gross Unrealized Gains	22,869,000		16,755,000
Gross Unrealized Losses	(67,842,000)	[1]	(101,387,000)	[1]
Fair Value	4,761,481,000		4,866,819,000
Percentage of foreign investments held in government and semi-government securities (in hundredths)	85.00%
Percentage of foreign investments held in corporate securities (in hundredths)	11.00%
Percentage of foreign investments held in cash equivalents (in hundredths)	4.00%
Percentage of Australian portfolio rated AAA	78.00%
Percentage of Australian portfolio rated AA	22.00%
Gross unrealized (gains) losses	67,800,000		(101,400,000)
Maturities, Amortized Cost [Abstract]
Due in one year or less	609,374,000
Due after one year through five years	1,842,541,000
Due after five years through ten years	779,379,000
Due after ten years	458,823,000
Total debt securities with single maturity date	3,690,117,000
Total at end of period	4,803,526,000		4,948,543,000
Maturities, Fair Value [Abstract]
Due in one year or less	610,697,000
Due after one year through five years	1,847,304,000
Due after five years through ten years	765,534,000
Due after ten years	444,960,000
Total debt securities with single maturity date	3,668,495,000
Total at end of period	4,758,526,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	2,299,646,000		2,687,628,000
12 months or greater	373,303,000		406,033,000
Total investment portfolio	2,672,949,000		3,093,661,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	45,779,000		74,583,000
12 months or greater	22,063,000		26,804,000
Total investment portfolio	67,842,000		101,387,000
U.S. Treasury securities and obligations of U.S. government corporations and agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	550,477,000		663,642,000
Gross Unrealized Gains	1,649,000		1,469,000
Gross Unrealized Losses	(17,078,000)	[1]	(25,521,000)	[1]
Fair Value	535,048,000		639,590,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	424,490,000		465,975,000
12 months or greater	18,043,000		4,103,000
Total investment portfolio	442,533,000		470,078,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	15,477,000		24,980,000
12 months or greater	1,601,000		541,000
Total investment portfolio	17,078,000		25,521,000
Obligations of U.S. states and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	906,385,000		932,922,000
Gross Unrealized Gains	8,232,000		5,865,000
Gross Unrealized Losses	(8,490,000)	[1]	(17,420,000)	[1]
Fair Value	906,127,000		921,367,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	377,446,000		503,967,000
12 months or greater	2,692,000		4,226,000
Total investment portfolio	380,138,000		508,193,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	8,445,000		17,370,000
12 months or greater	45,000		50,000
Total investment portfolio	8,490,000		17,420,000
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,192,333,000		2,190,095,000
Gross Unrealized Gains	9,038,000		6,313,000
Gross Unrealized Losses	(16,621,000)	[1]	(24,993,000)	[1]
Fair Value	2,184,750,000		2,171,415,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	1,072,046,000		1,238,211,000
12 months or greater	67,839,000		81,593,000
Total investment portfolio	1,139,885,000		1,319,804,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	13,570,000		20,371,000
12 months or greater	3,051,000		4,622,000
Total investment portfolio	16,621,000		24,993,000
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	392,435,000		399,839,000
Gross Unrealized Gains	1,317,000		1,100,000
Gross Unrealized Losses	(241,000)	[1]	(453,000)	[1]
Fair Value	393,511,000		400,486,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	392,435,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	393,511,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	88,241,000		126,991,000
12 months or greater	7,087,000		7,114,000
Total investment portfolio	95,328,000		134,105,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	185,000		387,000
12 months or greater	56,000		66,000
Total investment portfolio	241,000		453,000
Residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	373,314,000		383,368,000
Gross Unrealized Gains	141,000		146,000
Gross Unrealized Losses	(19,930,000)	[1]	(24,977,000)	[1]
Fair Value	353,525,000		358,537,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	373,314,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	353,525,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	90,098,000		91,534,000
12 months or greater	262,298,000		265,827,000
Total investment portfolio	352,396,000		357,361,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	2,765,000		3,886,000
12 months or greater	17,165,000		21,091,000
Total investment portfolio	19,930,000		24,977,000
Commercial mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	286,323,000		277,920,000
Gross Unrealized Gains	569,000		131,000
Gross Unrealized Losses	(4,331,000)	[1]	(6,668,000)	[1]
Fair Value	282,561,000		271,383,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	286,323,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	282,561,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	177,235,000		192,440,000
12 months or greater	15,256,000		43,095,000
Total investment portfolio	192,491,000		235,535,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	4,191,000		6,239,000
12 months or greater	140,000		429,000
Total investment portfolio	4,331,000		6,668,000
Collateralized loan obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	61,337,000		61,337,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	(903,000)	[1]	(1,042,000)	[1]
Fair Value	60,434,000		60,295,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	61,337,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	60,434,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	60,434,000		60,295,000
12 months or greater	0		0
Total investment portfolio	60,434,000		60,295,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	903,000		1,042,000
12 months or greater	0		0
Total investment portfolio	903,000		1,042,000
Debt securities issued by foreign sovereign governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	40,922,000		39,420,000
Gross Unrealized Gains	1,885,000		1,722,000
Gross Unrealized Losses	(237,000)	[1]	(290,000)	[1]
Fair Value	42,570,000		40,852,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	9,357,000		7,203,000
12 months or greater	0		0
Total investment portfolio	9,357,000		7,203,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	237,000		290,000
12 months or greater	0		0
Total investment portfolio	237,000		290,000
Total debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,803,526,000		4,948,543,000
Gross Unrealized Gains	22,831,000		16,746,000
Gross Unrealized Losses	(67,831,000)	[1]	(101,364,000)	[1]
Fair Value	4,758,526,000		4,863,925,000
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,928,000		2,908,000
Gross Unrealized Gains	38,000		9,000
Gross Unrealized Losses	(11,000)	[1]	(23,000)	[1]
Fair Value	2,955,000		2,894,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	299,000		1,012,000
12 months or greater	88,000		75,000
Total investment portfolio	387,000		1,087,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	6,000		18,000
12 months or greater	5,000		5,000
Total investment portfolio	$ 11,000		$ 23,000

[1] (1) At March 31, 2014 and December 31, 2013, there were no other-than-temporary impairment losses recorded in other comprehensive income.