Securities in an Unrealized Loss Position (Securities in an Unrealized Loss Position) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment				$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	$ 3,497,447,000	$ 4,560,889,000	$ 3,497,447,000
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	78,466,000	68,703,000	78,466,000
Available-for-sale Securities, 12 Months or Greater, Fair Value	672,929,000	599,999,000	672,929,000
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	35,482,000	30,565,000	35,482,000
Available-for-sale Securities, Fair Value	4,170,376,000	5,160,888,000	4,170,376,000
Available For Sale Securities, Gross Unrealized Losses	113,948,000	99,268,000	113,948,000
Fixed Income Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment			9,000,000
U.S. government and government agency
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	101,660,000	112,709,000	101,660,000
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	487,000	1,252,000	487,000
Available-for-sale Securities, 12 Months or Greater, Fair Value	64,500,000	35,450,000	64,500,000
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	1,846,000	1,341,000	1,846,000
Available-for-sale Securities, Fair Value	166,160,000	148,159,000	166,160,000
Available For Sale Securities, Gross Unrealized Losses	2,333,000	2,593,000	2,333,000
Held to maturity:
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	501,952,000	1,562,614,000	501,952,000
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	4,404,000	35,553,000	4,404,000
Available-for-sale Securities, 12 Months or Greater, Fair Value	106,681,000	133,034,000	106,681,000
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	1,865,000	4,885,000	1,865,000
Available-for-sale Securities, Fair Value	608,633,000	1,695,648,000	608,633,000
Available For Sale Securities, Gross Unrealized Losses	6,269,000	40,438,000	6,269,000
Mortgage-backed securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	381,986,000	625,903,000	381,986,000
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	3,639,000	11,103,000	3,639,000
Available-for-sale Securities, 12 Months or Greater, Fair Value	184,807,000	109,066,000	184,807,000
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	7,669,000	4,828,000	7,669,000
Available-for-sale Securities, Fair Value	566,793,000	734,969,000	566,793,000
Available For Sale Securities, Gross Unrealized Losses	11,308,000	15,931,000	11,308,000
Asset-backed Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	1,091,078,000	1,010,836,000	1,091,078,000
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	7,703,000	5,340,000	7,703,000
Available-for-sale Securities, 12 Months or Greater, Fair Value	190,467,000	201,693,000	190,467,000
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	6,711,000	6,601,000	6,711,000
Available-for-sale Securities, Fair Value	1,281,545,000	1,212,529,000	1,281,545,000
Available For Sale Securities, Gross Unrealized Losses	14,414,000	11,941,000	14,414,000
Residential mortgage-backed
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	1,232,940,000	1,035,245,000	1,232,940,000
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	35,406,000	13,448,000	35,406,000
Available-for-sale Securities, 12 Months or Greater, Fair Value	76,797,000	65,147,000	76,797,000
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	8,987,000	7,470,000	8,987,000
Available-for-sale Securities, Fair Value	1,309,737,000	1,100,392,000	1,309,737,000
Available For Sale Securities, Gross Unrealized Losses	44,393,000	20,918,000	44,393,000
Corporate backed
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	169,190,000	213,246,000	169,190,000
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	8,822,000	1,985,000	8,822,000
Available-for-sale Securities, 12 Months or Greater, Fair Value	19,528,000	24,820,000	19,528,000
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	3,867,000	777,000	3,867,000
Available-for-sale Securities, Fair Value	188,718,000	238,066,000	188,718,000
Available For Sale Securities, Gross Unrealized Losses	12,689,000	2,762,000	12,689,000
Fixed maturity securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	3,478,806,000	4,560,553,000	3,478,806,000
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	60,461,000	68,681,000	60,461,000
Available-for-sale Securities, 12 Months or Greater, Fair Value	642,780,000	569,210,000	642,780,000
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	30,945,000	25,902,000	30,945,000
Available-for-sale Securities, Fair Value	4,121,586,000	5,129,763,000	4,121,586,000
Available For Sale Securities, Gross Unrealized Losses	91,406,000	94,583,000	91,406,000
Common stocks
Schedule of Available-for-sale Securities [Line Items]
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment		18,100,000	10,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	18,641,000	336,000	18,641,000
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	18,005,000	22,000	18,005,000
Available-for-sale Securities, 12 Months or Greater, Fair Value	7,829,000	8,755,000	7,829,000
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	1,184,000	1,024,000	1,184,000
Available-for-sale Securities, Fair Value	26,470,000	9,091,000	26,470,000
Available For Sale Securities, Gross Unrealized Losses	19,189,000	$ 1,046,000	19,189,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Other		two
Preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment	13,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	0	$ 0	0
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	0	0	0
Available-for-sale Securities, 12 Months or Greater, Fair Value	22,320,000	22,034,000	22,320,000
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	3,353,000	3,639,000	3,353,000
Available-for-sale Securities, Fair Value	22,320,000	22,034,000	22,320,000
Available For Sale Securities, Gross Unrealized Losses	3,353,000	$ 3,639,000	3,353,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Other		one
Equity securities available for sale
Schedule of Available-for-sale Securities [Line Items]
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment		$ 18,100,000	24,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	18,641,000	336,000	18,641,000
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	18,005,000	22,000	18,005,000
Available-for-sale Securities, 12 Months or Greater, Fair Value	30,149,000	30,789,000	30,149,000
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	4,537,000	4,663,000	4,537,000
Available-for-sale Securities, Fair Value	48,790,000	31,125,000	48,790,000
Available For Sale Securities, Gross Unrealized Losses	$ 22,542,000	$ 4,685,000	$ 22,542,000