Investments (Details) (USD $)	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 4,951,451,000		$ 4,188,734,000
Gross Unrealized Gains	16,755,000		46,793,000
Gross Unrealized Losses	(101,387,000)	[1]	(5,252,000)	[1]
Fair Value	4,866,819,000		4,230,275,000
Percentage of foreign investments held in government and semi-government securities (in hundredths)	84.00%
Percentage of foreign investments held in corporate securities (in hundredths)	11.00%
Percentage Of Foreign Investments Held In Cash equivalents (in hundredths)	5.00%
Repatriation of capital to parent	89,500,000
Equity Value in Australian Operations	46,000,000
Gross unrealized losses	101,400,000		5,300,000
Maturities, Amortized Cost [Abstract]
Due in one year or less	739,401,000
Due after one year through five years	1,800,785,000
Due after five years through ten years	809,680,000
Due after ten years	476,213,000
Total debt securities with single maturity date	3,826,079,000
Total at end of period	4,948,543,000		4,185,937,000
Maturities, Fair Value [Abstract]
Due in one year or less	740,155,000
Due after one year through five years	1,800,866,000
Due after five years through ten years	781,170,000
Due after ten years	451,033,000
Total debt securities with single maturity date	3,773,224,000
Total at end of period	4,863,925,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	2,687,628,000		893,029,000
12 months or greater	406,033,000		24,298,000
Total investment portfolio	3,093,661,000		917,327,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	74,583,000		4,917,000
12 months or greater	26,804,000		335,000
Total investment portfolio	101,387,000		5,252,000
Percentage of Australian portfolio rated AAA	78.00%
Percentage of Australian portfolio rated AA	22.00%
For securities in an unrealized loss position, percentage of weighted average fair value to amortized cost (in hundredths)	97.00%
For securities in an unrealized loss position, percentage backed by the U.S. Government (in hundredths)	50.00%
Recognized other than temporary impairments	328,000		2,310,000		715,000
Investments on deposit with various states	20,300,000		21,400,000
U.S. Treasury securities and obligations of U.S. government corporations and agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	663,642,000		863,282,000
Gross Unrealized Gains	1,469,000		3,040,000
Gross Unrealized Losses	(25,521,000)	[1]	(71,000)	[1]
Fair Value	639,590,000		866,251,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	465,975,000		24,094,000
12 months or greater	4,103,000		0
Total investment portfolio	470,078,000		24,094,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	24,980,000		71,000
12 months or greater	541,000		0
Total investment portfolio	25,521,000		71,000
Obligations of U.S. States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	932,922,000		795,935,000
Gross Unrealized Gains	5,865,000		16,965,000
Gross Unrealized Losses	(17,420,000)	[1]	(506,000)	[1]
Fair Value	921,367,000		812,394,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	503,967,000		156,111,000
12 months or greater	4,226,000		1,006,000
Total investment portfolio	508,193,000		157,117,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	17,370,000		505,000
12 months or greater	50,000		1,000
Total investment portfolio	17,420,000		506,000
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,190,095,000		1,469,844,000
Gross Unrealized Gains	6,313,000		13,813,000
Gross Unrealized Losses	(24,993,000)	[1]	(2,716,000)	[1]
Fair Value	2,171,415,000		1,480,941,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	1,238,211,000		280,765,000
12 months or greater	81,593,000		3,353,000
Total investment portfolio	1,319,804,000		284,118,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	20,371,000		2,714,000
12 months or greater	4,622,000		2,000
Total investment portfolio	24,993,000		2,716,000
Residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	383,368,000		451,352,000
Gross Unrealized Gains	146,000		871,000
Gross Unrealized Losses	(24,977,000)	[1]	(1,314,000)	[1]
Fair Value	358,537,000		450,909,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	383,368,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	358,537,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	91,534,000		315,000,000
12 months or greater	265,827,000		19,939,000
Total investment portfolio	357,361,000		334,939,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	3,886,000		982,000
12 months or greater	21,091,000		332,000
Total investment portfolio	24,977,000		1,314,000
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	277,920,000		150,232,000
Gross Unrealized Gains	131,000		524,000
Gross Unrealized Losses	(6,668,000)	[1]	(414,000)	[1]
Fair Value	271,383,000		150,342,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	277,920,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	271,383,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	192,440,000		72,689,000
12 months or greater	43,095,000		0
Total investment portfolio	235,535,000		72,689,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	6,239,000		414,000
12 months or greater	429,000		0
Total investment portfolio	6,668,000		414,000
Debt securities issued by foreign sovereign governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	39,420,000		132,490,000
Gross Unrealized Gains	1,722,000		9,784,000
Gross Unrealized Losses	(290,000)	[1]	(208,000)	[1]
Fair Value	40,852,000		142,066,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	7,203,000		14,695,000
12 months or greater	0		0
Total investment portfolio	7,203,000		14,695,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	290,000		208,000
12 months or greater	0		0
Total investment portfolio	290,000		208,000
Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,948,543,000		4,185,937,000
Gross Unrealized Gains	16,746,000		46,654,000
Gross Unrealized Losses	(101,364,000)	[1]	(5,252,000)	[1]
Fair Value	4,863,925,000		4,227,339,000
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,908,000		2,797,000
Gross Unrealized Gains	9,000		139,000
Gross Unrealized Losses	(23,000)	[1]	0	[1]
Fair Value	2,894,000		2,936,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	1,012,000
12 months or greater	75,000
Total investment portfolio	1,087,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	18,000
12 months or greater	5,000
Total investment portfolio	23,000
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	399,839,000		322,802,000
Gross Unrealized Gains	1,100,000		1,657,000
Gross Unrealized Losses	(453,000)	[1]	(23,000)	[1]
Fair Value	400,486,000		324,436,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	399,839,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	400,486,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	126,991,000		29,675,000
12 months or greater	7,114,000		0
Total investment portfolio	134,105,000		29,675,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	387,000		23,000
12 months or greater	66,000		0
Total investment portfolio	453,000		23,000
Collateralized Loan Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	61,337,000
Gross Unrealized Gains	0
Gross Unrealized Losses	(1,042,000)	[1]
Fair Value	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	60,295,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	60,295,000
12 months or greater	0
Total investment portfolio	60,295,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	1,042,000
12 months or greater	0
Total investment portfolio	$ 1,042,000

[1] There were no other-than-temporary impairment losses recorded in other comprehensive income at December 31, 2013 and 2012.