Investments - Maturity Table (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,081,236	$ 485,756
Estimated Fair Value	2,090,707	$ 487,669
2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	214,950
Estimated Fair Value	214,970
2021
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	454,287
Estimated Fair Value	455,932
2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	878,920
Estimated Fair Value	880,483
2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	139,699
Estimated Fair Value	140,147
2038
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	172
Estimated Fair Value	195
2039
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,058
Estimated Fair Value	2,271
2042
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,738
Estimated Fair Value	5,750
2043
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,572
Estimated Fair Value	10,027
2044
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	12,993
Estimated Fair Value	13,540
2045
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	12,176
Estimated Fair Value	12,611
2046
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	19,044
Estimated Fair Value	19,485
2047
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	28,940
Estimated Fair Value	29,448
2048
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,907
Estimated Fair Value	7,209
2049
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	56,060
Estimated Fair Value	58,279
2050
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	167,454
Estimated Fair Value	168,041
2054
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	72,266
Estimated Fair Value	$ 72,319