Fixed Maturity Securities In An Unrealized Loss Position (Securities in an Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	$ 6,402,677	$ 3,783,769
Gross Unrealized Losses, Less than 12 Months	120,766	31,682
Fair Value, 12 Months or Greater	1,107,459	1,155,860
Gross Unrealized Losses, 12 Months or Greater	38,761	28,547
Fair Value, Total	7,510,136	4,939,629
Gross Unrealized Losses, Total	159,527	60,229
U.S. government and government agency
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	245,718	92,167
Gross Unrealized Losses, Less than 12 Months	2,852	1,491
Fair Value, 12 Months or Greater	93,232	72,055
Gross Unrealized Losses, 12 Months or Greater	3,411	2,341
Fair Value, Total	338,950	164,222
Gross Unrealized Losses, Total	6,263	3,832
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	1,406,792	735,972
Gross Unrealized Losses, Less than 12 Months	22,540	5,944
Fair Value, 12 Months or Greater	302,403	345,755
Gross Unrealized Losses, 12 Months or Greater	9,679	6,630
Fair Value, Total	1,709,195	1,081,727
Gross Unrealized Losses, Total	32,219	12,574
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	859,133	480,435
Gross Unrealized Losses, Less than 12 Months	17,171	5,110
Fair Value, 12 Months or Greater	328,074	373,956
Gross Unrealized Losses, 12 Months or Greater	14,699	11,065
Fair Value, Total	1,187,207	854,391
Gross Unrealized Losses, Total	31,870	16,175
Asset-backed
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	1,538,222	1,127,309
Gross Unrealized Losses, Less than 12 Months	12,216	8,298
Fair Value, 12 Months or Greater	160,722	167,412
Gross Unrealized Losses, 12 Months or Greater	868	2,314
Fair Value, Total	1,698,944	1,294,721
Gross Unrealized Losses, Total	13,084	10,612
Corporate
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	2,171,776	1,103,747
Gross Unrealized Losses, Less than 12 Months	61,233	8,224
Fair Value, 12 Months or Greater	155,159	170,858
Gross Unrealized Losses, 12 Months or Greater	8,988	5,946
Fair Value, Total	2,326,935	1,274,605
Gross Unrealized Losses, Total	70,221	14,170
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	181,036	244,139
Gross Unrealized Losses, Less than 12 Months	4,754	2,615
Fair Value, 12 Months or Greater	67,869	25,824
Gross Unrealized Losses, 12 Months or Greater	1,116	251
Fair Value, Total	248,905	269,963
Gross Unrealized Losses, Total	$ 5,870	$ 2,866