Investments - Maturity Table (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 607,867	$ 485,756
Estimated Fair Value	614,582	$ 487,669
2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	94,048
Estimated Fair Value	94,320
2021
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	145,340
Estimated Fair Value	147,711
2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	54,127
Estimated Fair Value	54,317
2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	12,977
Estimated Fair Value	13,341
2038
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	175
Estimated Fair Value	192
2039
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,457
Estimated Fair Value	2,668
2042
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,115
Estimated Fair Value	7,172
2043
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	11,130
Estimated Fair Value	11,560
2044
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	16,149
Estimated Fair Value	16,684
2045
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	16,896
Estimated Fair Value	17,286
2046
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	27,641
Estimated Fair Value	28,385
2047
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	39,927
Estimated Fair Value	40,313
2048
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,248
Estimated Fair Value	10,664
2049
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	84,282
Estimated Fair Value	87,458
2050
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,550
Estimated Fair Value	6,671
2054
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	78,805
Estimated Fair Value	$ 75,840