Securities in an Unrealized Loss Position (Securities in an Unrealized Loss Position) (Details)	12 Months Ended
	Dec. 31, 2015 USD ($) position	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment	$ 33,300,000	$ 0	$ 6,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	3,497,447,000	2,024,847,000
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	78,466,000	20,681,000
Available-for-sale Securities, 12 Months or Greater, Fair Value	672,929,000	870,357,000
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	35,482,000	48,461,000
Available-for-sale Securities, Fair Value	4,170,376,000	2,895,204,000
Available For Sale Securities, Gross Unrealized Losses	113,948,000	69,142,000
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	0
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	84,750,000
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	487,000	522,000
Available-for-sale Securities, 12 Months or Greater, Fair Value	64,500,000	84,850,000
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	1,846,000	2,635,000
Available-for-sale Securities, Fair Value	166,160,000	169,600,000
Available For Sale Securities, Gross Unrealized Losses	2,333,000	3,157,000
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	158,594,000
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	4,404,000	631,000
Available-for-sale Securities, 12 Months or Greater, Fair Value	106,681,000	150,284,000
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	1,865,000	3,389,000
Available-for-sale Securities, Fair Value	608,633,000	308,878,000
Available For Sale Securities, Gross Unrealized Losses	6,269,000	4,020,000
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	75,739,000
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	3,639,000	332,000
Available-for-sale Securities, 12 Months or Greater, Fair Value	184,807,000	312,922,000
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	7,669,000	9,169,000
Available-for-sale Securities, Fair Value	566,793,000	388,661,000
Available For Sale Securities, Gross Unrealized Losses	11,308,000	9,501,000
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	1,186,097,000
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	7,703,000	6,217,000
Available-for-sale Securities, 12 Months or Greater, Fair Value	190,467,000	30,818,000
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	6,711,000	5,757,000
Available-for-sale Securities, Fair Value	1,281,545,000	1,216,915,000
Available For Sale Securities, Gross Unrealized Losses	14,414,000	11,974,000
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	400,141,000
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	35,406,000	2,480,000
Available-for-sale Securities, 12 Months or Greater, Fair Value	76,797,000	183,810,000
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	8,987,000	10,327,000
Available-for-sale Securities, Fair Value	1,309,737,000	583,951,000
Available For Sale Securities, Gross Unrealized Losses	44,393,000	12,807,000
Corporate backed
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	169,190,000	76,471,000
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	8,822,000	3,907,000
Available-for-sale Securities, 12 Months or Greater, Fair Value	19,528,000	85,025,000
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	3,867,000	14,158,000
Available-for-sale Securities, Fair Value	188,718,000	161,496,000
Available For Sale Securities, Gross Unrealized Losses	12,689,000	18,065,000
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	1,981,792,000
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	60,461,000	14,089,000
Available-for-sale Securities, 12 Months or Greater, Fair Value	642,780,000	847,709,000
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	30,945,000	45,435,000
Available-for-sale Securities, Fair Value	4,121,586,000	2,829,501,000
Available For Sale Securities, Gross Unrealized Losses	91,406,000	59,524,000
Common stocks
Schedule of Available-for-sale Securities [Line Items]
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment	$ 10,900,000	0
Number Of Securities | position	2
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	$ 18,641,000	15,929,000
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	18,005,000	5,453,000
Available-for-sale Securities, 12 Months or Greater, Fair Value	7,829,000	0
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	1,184,000	0
Available-for-sale Securities, Fair Value	26,470,000	15,929,000
Available For Sale Securities, Gross Unrealized Losses	19,189,000	5,453,000
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	0
Number Of Securities | position	1
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	$ 0	27,126,000
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	0	1,139,000
Available-for-sale Securities, 12 Months or Greater, Fair Value	22,320,000	22,648,000
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	3,353,000	3,026,000
Available-for-sale Securities, Fair Value	22,320,000	49,774,000
Available For Sale Securities, Gross Unrealized Losses	3,353,000	4,165,000
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	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	43,055,000
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	18,005,000	6,592,000
Available-for-sale Securities, 12 Months or Greater, Fair Value	30,149,000	22,648,000
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	4,537,000	3,026,000
Available-for-sale Securities, Fair Value	48,790,000	65,703,000
Available For Sale Securities, Gross Unrealized Losses	$ 22,542,000	$ 9,618,000