Investments (Available-For-Sale Fixed Maturities) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Reclassifications from AOCI to net income (loss):
Amortized Cost	$ 2,287,956	$ 2,241,304
Allowance for Expected Credit Losses	1,163	0
Gross Unrealized Gains	45,791	50,565
Gross Unrealized Losses	50,920	3,084
Estimated Fair Value	2,281,664	2,288,785
U.S. Treasury obligations
Reclassifications from AOCI to net income (loss):
Amortized Cost	123,840	109,060
Allowance for Expected Credit Losses	0
Gross Unrealized Gains	5,420	1,533
Gross Unrealized Losses	0	126
Estimated Fair Value	129,260	110,467
U.S. Government-sponsored enterprise obligations
Reclassifications from AOCI to net income (loss):
Amortized Cost	12,205	17,215
Allowance for Expected Credit Losses	0
Gross Unrealized Gains	205	125
Gross Unrealized Losses	0	0
Estimated Fair Value	12,410	17,340
State and municipal bonds
Reclassifications from AOCI to net income (loss):
Amortized Cost	282,451	287,658
Allowance for Expected Credit Losses	0
Gross Unrealized Gains	9,795	9,110
Gross Unrealized Losses	579	675
Estimated Fair Value	291,667	296,093
Corporate debt
Reclassifications from AOCI to net income (loss):
Amortized Cost	1,302,979	1,308,889
Allowance for Expected Credit Losses	1,163	0
Gross Unrealized Gains	20,157	33,050
Gross Unrealized Losses	33,255	1,575
Estimated Fair Value	1,288,718	1,340,364
Residential mortgage-backed securities
Reclassifications from AOCI to net income (loss):
Amortized Cost	228,584	205,588
Allowance for Expected Credit Losses	0
Gross Unrealized Gains	7,174	3,139
Gross Unrealized Losses	5,160	319
Estimated Fair Value	230,598	208,408
Agency commercial mortgage-backed securities
Reclassifications from AOCI to net income (loss):
Amortized Cost	12,901	8,054
Allowance for Expected Credit Losses	0
Gross Unrealized Gains	464	182
Gross Unrealized Losses	2	15
Estimated Fair Value	13,363	8,221
Residential mortgage-backed and other commercial mortgage-backed securities
Reclassifications from AOCI to net income (loss):
Amortized Cost	80,470	70,621
Allowance for Expected Credit Losses	0
Gross Unrealized Gains	1,446	1,468
Gross Unrealized Losses	2,792	221
Estimated Fair Value	79,124	71,868
Other asset-backed securities
Reclassifications from AOCI to net income (loss):
Amortized Cost	244,526	234,219
Allowance for Expected Credit Losses	0
Gross Unrealized Gains	1,130	1,958
Gross Unrealized Losses	9,132	153
Estimated Fair Value	$ 236,524	$ 236,024