Securities in an Unrealized Loss Position (Securities in an Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	$ 3,793,013	$ 4,560,889
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	32,893	68,703
Available-for-sale Securities, 12 Months or Greater, Fair Value	1,188,324	599,999
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	31,893	30,565
Available-for-sale Securities, Fair Value	4,981,337	5,160,888
Available For Sale Securities, Gross Unrealized Losses	64,786	99,268
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	92,167	112,709
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	1,491	1,252
Available-for-sale Securities, 12 Months or Greater, Fair Value	72,055	35,450
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	2,341	1,341
Available-for-sale Securities, Fair Value	164,222	148,159
Available For Sale Securities, Gross Unrealized Losses	3,832	2,593
State and municipal
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	735,972	1,562,614
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	5,944	35,553
Available-for-sale Securities, 12 Months or Greater, Fair Value	345,755	133,034
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	6,630	4,885
Available-for-sale Securities, Fair Value	1,081,727	1,695,648
Available For Sale Securities, Gross Unrealized Losses	12,574	40,438
Mortgage-backed securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	480,435	625,903
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	5,110	11,103
Available-for-sale Securities, 12 Months or Greater, Fair Value	373,956	109,066
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	11,065	4,828
Available-for-sale Securities, Fair Value	854,391	734,969
Available For Sale Securities, Gross Unrealized Losses	16,175	15,931
Asset-backed Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	1,127,309	1,010,836
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	8,298	5,340
Available-for-sale Securities, 12 Months or Greater, Fair Value	167,412	201,693
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	2,314	6,601
Available-for-sale Securities, Fair Value	1,294,721	1,212,529
Available For Sale Securities, Gross Unrealized Losses	10,612	11,941
Total corporate
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	1,103,747	1,035,245
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	8,224	13,448
Available-for-sale Securities, 12 Months or Greater, Fair Value	170,858	65,147
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	5,946	7,470
Available-for-sale Securities, Fair Value	1,274,605	1,100,392
Available For Sale Securities, Gross Unrealized Losses	14,170	20,918
Foreign government
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	244,139	213,246
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	2,615	1,985
Available-for-sale Securities, 12 Months or Greater, Fair Value	25,824	24,820
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	251	777
Available-for-sale Securities, Fair Value	269,963	238,066
Available For Sale Securities, Gross Unrealized Losses	2,866	2,762
Fixed maturity securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	3,783,769	4,560,553
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	31,682	68,681
Available-for-sale Securities, 12 Months or Greater, Fair Value	1,155,860	569,210
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	28,547	25,902
Available-for-sale Securities, Fair Value	4,939,629	5,129,763
Available For Sale Securities, Gross Unrealized Losses	60,229	94,583
Common stocks
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	9,244	336
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	1,211	22
Available-for-sale Securities, 12 Months or Greater, Fair Value	9,387	8,755
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	749	1,024
Available-for-sale Securities, Fair Value	18,631	9,091
Available For Sale Securities, Gross Unrealized Losses	1,960	1,046
Preferred stocks
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	0	0
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	0	0
Available-for-sale Securities, 12 Months or Greater, Fair Value	23,077	22,034
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	2,597	3,639
Available-for-sale Securities, Fair Value	23,077	22,034
Available For Sale Securities, Gross Unrealized Losses	2,597	3,639
Equity securities available for sale
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	9,244	336
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	1,211	22
Available-for-sale Securities, 12 Months or Greater, Fair Value	32,464	30,789
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	3,346	4,663
Available-for-sale Securities, Fair Value	41,708	31,125
Available For Sale Securities, Gross Unrealized Losses	$ 4,557	$ 4,685