Investments (Details) (USD $)	9 Months Ended		12 Months Ended
	Sep. 30, 2014		Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 4,647,529,000		$ 4,951,451,000
Gross Unrealized Gains	26,960,000		16,755,000
Gross Unrealized Losses	(44,488,000)	[1]	(101,387,000)	[1]
Fair Value	4,630,001,000		4,866,819,000
Percentage of foreign investments held in government and semi-government securities (in hundredths)	86.00%
Percentage of foreign investments held in corporate securities (in hundredths)	10.00%
Percentage of foreign investments held in cash equivalents (in hundredths)	4.00%
Percentage of Australian portfolio rated AAA (in hundredths)	79.00%
Percentage of Australian portfolio rated AA (in hundredths)	21.00%
Gross unrealized (gains) losses	44,500,000		101,400,000
Maturities, Amortized Cost [Abstract]
Due in one year or less	405,324,000
Due after one year through five years	1,858,612,000
Due after five years through ten years	928,153,000
Due after ten years	435,242,000
Total debt securities with single maturity date	3,627,331,000
Total at end of period	4,644,561,000		4,948,543,000
Maturities, Fair Value [Abstract]
Due in one year or less	406,256,000
Due after one year through five years	1,863,872,000
Due after five years through ten years	922,372,000
Due after ten years	433,223,000
Total debt securities with single maturity date	3,625,723,000
Total at end of period	4,626,979,000		4,863,925,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	1,340,870,000		2,687,628,000
12 months or greater	1,190,817,000		406,033,000
Total investment portfolio	2,531,687,000		3,093,661,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	8,488,000		74,583,000
12 months or greater	36,000,000		26,804,000
Total investment portfolio	44,488,000		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	414,778,000		663,642,000
Gross Unrealized Gains	2,636,000		1,469,000
Gross Unrealized Losses	(9,795,000)	[1]	(25,521,000)	[1]
Fair Value	407,619,000		639,590,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	6,320,000		465,975,000
12 months or greater	325,892,000		4,103,000
Total investment portfolio	332,212,000		470,078,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	43,000		24,980,000
12 months or greater	9,752,000		541,000
Total investment portfolio	9,795,000		25,521,000
Obligations of U.S. states and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	833,510,000		932,922,000
Gross Unrealized Gains	10,068,000		5,865,000
Gross Unrealized Losses	(4,316,000)	[1]	(17,420,000)	[1]
Fair Value	839,262,000		921,367,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	165,975,000		503,967,000
12 months or greater	135,146,000		4,226,000
Total investment portfolio	301,121,000		508,193,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	1,392,000		17,370,000
12 months or greater	2,924,000		50,000
Total investment portfolio	4,316,000		17,420,000
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,341,189,000		2,190,095,000
Gross Unrealized Gains	9,953,000		6,313,000
Gross Unrealized Losses	(12,587,000)	[1]	(24,993,000)	[1]
Fair Value	2,338,555,000		2,171,415,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	1,000,551,000		1,238,211,000
12 months or greater	233,161,000		81,593,000
Total investment portfolio	1,233,712,000		1,319,804,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	5,995,000		20,371,000
12 months or greater	6,592,000		4,622,000
Total investment portfolio	12,587,000		24,993,000
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	341,519,000		399,839,000
Gross Unrealized Gains	932,000		1,100,000
Gross Unrealized Losses	(202,000)	[1]	(453,000)	[1]
Fair Value	342,249,000		400,486,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	341,519,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	342,249,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	56,801,000		126,991,000
12 months or greater	14,675,000		7,114,000
Total investment portfolio	71,476,000		134,105,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	137,000		387,000
12 months or greater	65,000		66,000
Total investment portfolio	202,000		453,000
Residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	344,515,000		383,368,000
Gross Unrealized Gains	167,000		146,000
Gross Unrealized Losses	(13,751,000)	[1]	(24,977,000)	[1]
Fair Value	330,931,000		358,537,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	344,515,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	330,931,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	15,020,000		91,534,000
12 months or greater	307,151,000		265,827,000
Total investment portfolio	322,171,000		357,361,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	55,000		3,886,000
12 months or greater	13,696,000		21,091,000
Total investment portfolio	13,751,000		24,977,000
Commercial mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	269,857,000		277,920,000
Gross Unrealized Gains	675,000		131,000
Gross Unrealized Losses	(3,032,000)	[1]	(6,668,000)	[1]
Fair Value	267,500,000		271,383,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	269,857,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	267,500,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	91,505,000		192,440,000
12 months or greater	112,195,000		43,095,000
Total investment portfolio	203,700,000		235,535,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	855,000		6,239,000
12 months or greater	2,177,000		429,000
Total investment portfolio	3,032,000		6,668,000
Collateralized loan obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	61,339,000		61,337,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	(763,000)	[1]	(1,042,000)	[1]
Fair Value	60,576,000		60,295,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	61,339,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	60,576,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	0		60,295,000
12 months or greater	60,576,000		0
Total investment portfolio	60,576,000		60,295,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	0		1,042,000
12 months or greater	763,000		0
Total investment portfolio	763,000		1,042,000
Debt securities issued by foreign sovereign governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	37,854,000		39,420,000
Gross Unrealized Gains	2,466,000		1,722,000
Gross Unrealized Losses	(33,000)	[1]	(290,000)	[1]
Fair Value	40,287,000		40,852,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	4,561,000		7,203,000
12 months or greater	1,783,000		0
Total investment portfolio	6,344,000		7,203,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	10,000		290,000
12 months or greater	23,000		0
Total investment portfolio	33,000		290,000
Total debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,644,561,000		4,948,543,000
Gross Unrealized Gains	26,897,000		16,746,000
Gross Unrealized Losses	(44,479,000)	[1]	(101,364,000)	[1]
Fair Value	4,626,979,000		4,863,925,000
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,968,000		2,908,000
Gross Unrealized Gains	63,000		9,000
Gross Unrealized Losses	(9,000)	[1]	(23,000)	[1]
Fair Value	3,022,000		2,894,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	137,000		1,012,000
12 months or greater	238,000		75,000
Total investment portfolio	375,000		1,087,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	1,000		18,000
12 months or greater	8,000		5,000
Total investment portfolio	$ 9,000		$ 23,000

[1] At September 30, 2014 and December 31, 2013, there were no other-than-temporary impairment losses recorded in other comprehensive income.