Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	$ 5,672,315	$ 5,456,980
Fair Value	6,168,558	5,791,676
FNMA
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	390,700	405,100
FHLMC
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	322,400	283,100
GNMA
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	137,400	147,400
Total Fixed Maturity Securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	5,672,315	5,456,980
Unrealized Gains	541,202	348,310
Unrealized Losses	44,959	13,614
Fair Value	6,168,558	5,791,676
Mortgage-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	617,856	684,543
Unrealized Gains	85,219	41,263
Unrealized Losses	213	1,487
Fair Value	702,862	724,319
Other, including U.S. Treasury securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	341,771	436,665
Unrealized Gains	42,683	22,824
Unrealized Losses	317	621
Fair Value	384,137	458,868
Municipal bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	1,621,308	1,545,787
Unrealized Gains	196,317	141,996
Unrealized Losses	1,124	1,580
Fair Value	1,816,501	1,686,203
Foreign government bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	40,143	42,801
Unrealized Gains	4,644	2,569
Unrealized Losses	0	0
Fair Value	44,787	45,370
Corporate bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	1,802,423	1,464,444
Unrealized Gains	189,986	118,775
Unrealized Losses	8,926	1,795
Fair Value	1,983,483	1,581,424
Other asset-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	1,248,814	1,282,740
Unrealized Gains	22,353	20,883
Unrealized Losses	34,379	8,131
Fair Value	$ 1,236,788	$ 1,295,492

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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