Investment Operations Available for Sale Securities (Details) - USD ($) $ in Thousands		Dec. 31, 2019	Dec. 31, 2018
Debt Securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		$ 7,015,269	$ 6,856,277
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		709,717	305,709
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		22,358	128,941
Fair Value		7,702,628	7,033,045
Non-credit losses on other-than-temporary impairments (1)	[1]	2,499	3,966
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		3,376,432	3,231,846
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		418,049	138,972
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		15,531	90,933
Fair Value		3,778,950	3,279,885
Non-credit losses on other-than-temporary impairments (1)		0	0
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		626,663	584,133
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		47,654	29,969
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		1,929	7,242
Fair Value		672,388	606,860
Non-credit losses on other-than-temporary impairments (1)	[1]	1,997	2,823
Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		969,453	873,672
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		77,433	24,284
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		1,413	19,390
Fair Value		1,045,473	878,566
Non-credit losses on other-than-temporary impairments (1)		0	0
Other asset-backed
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		697,390	697,332
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		19,745	15,567
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		2,614	5,329
Fair Value		714,521	707,570
Non-credit losses on other-than-temporary impairments (1)	[1]	502	1,143
United States Government and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		12,417	19,673
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		1,711	996
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		5	134
Fair Value		14,123	20,535
Non-credit losses on other-than-temporary impairments (1)		0	0
States and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		1,332,914	1,449,621
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		145,125	95,921
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		866	5,913
Fair Value		1,477,173	1,539,629
Non-credit losses on other-than-temporary impairments (1)		$ 0	$ 0

[1] Non-credit losses subsequent to the initial impairment measurement date on OTTI losses are included in the gross unrealized gains and gross unrealized losses columns above. The non-credit loss component of OTTI losses for residential mortgage-backed and other asset-backed securities at December 31, 2019 and December 31, 2018 were in an unrealized gain position due to increases in estimated fair value subsequent to initial recognition of non-credit losses on such securities.