Securities in an Unrealized Loss Position (Securities in an Unrealized Loss Position) (Details) - External Credit Rating, Investment Grade - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
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	$ 195,359	$ 92,167
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	933	1,491
Available-for-sale Securities, 12 Months or Greater, Fair Value	130,815	72,055
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	3,977	2,341
Available-for-sale Securities, Fair Value	326,174	164,222
Available For Sale Securities, Gross Unrealized Losses	4,910	3,832
State and municipal
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	701,700	735,972
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	6,874	5,944
Available-for-sale Securities, 12 Months or Greater, Fair Value	744,905	345,755
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	17,738	6,630
Available-for-sale Securities, Fair Value	1,446,605	1,081,727
Available For Sale Securities, Gross Unrealized Losses	24,612	12,574
Mortgage-backed securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	334,063	480,435
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	2,911	5,110
Available-for-sale Securities, 12 Months or Greater, Fair Value	712,595	373,956
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	21,022	11,065
Available-for-sale Securities, Fair Value	1,046,658	854,391
Available For Sale Securities, Gross Unrealized Losses	23,933	16,175
Asset-backed securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	1,687,665	1,127,309
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	28,965	8,298
Available-for-sale Securities, 12 Months or Greater, Fair Value	342,855	167,412
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	4,722	2,314
Available-for-sale Securities, Fair Value	2,030,520	1,294,721
Available For Sale Securities, Gross Unrealized Losses	33,687	10,612
Total corporate
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	1,730,513	1,103,747
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	54,181	8,224
Available-for-sale Securities, 12 Months or Greater, Fair Value	954,763	170,858
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	49,129	5,946
Available-for-sale Securities, Fair Value	2,685,276	1,274,605
Available For Sale Securities, Gross Unrealized Losses	103,310	14,170
Foreign government
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	246,273	244,139
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	24,197	2,615
Available-for-sale Securities, 12 Months or Greater, Fair Value	80,004	25,824
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	914	251
Available-for-sale Securities, Fair Value	326,277	269,963
Available For Sale Securities, Gross Unrealized Losses	25,111	2,866
Fixed maturity securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	4,895,573	3,783,769
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	118,061	31,682
Available-for-sale Securities, 12 Months or Greater, Fair Value	2,965,937	1,155,860
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	97,502	28,547
Available-for-sale Securities, Fair Value	7,861,510	4,939,629
Available For Sale Securities, Gross Unrealized Losses	$ 215,563	$ 60,229