Investments - Maturity Table (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,986,957	$ 485,756
Estimated Fair Value	1,996,634	$ 487,669
Maturity 2021
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	454,193
Estimated Fair Value	455,185
Maturity 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	978,017
Estimated Fair Value	979,802
Maturity 2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	164,680
Estimated Fair Value	165,299
Maturity 2038
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	170
Estimated Fair Value	195
Maturity 2039
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,032
Estimated Fair Value	2,249
Maturity 2042
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,767
Estimated Fair Value	4,769
Maturity 2043
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,745
Estimated Fair Value	9,153
Maturity 2044
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	11,586
Estimated Fair Value	12,139
Maturity 2045
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,739
Estimated Fair Value	10,133
Maturity 2046
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	15,292
Estimated Fair Value	15,748
Maturity 2047
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	25,056
Estimated Fair Value	25,574
Maturity 2048
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,466
Estimated Fair Value	6,743
Maturity 2049
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	44,711
Estimated Fair Value	46,681
Maturity 2050
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	192,706
Estimated Fair Value	194,056
Maturity 2054
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	68,797
Estimated Fair Value	$ 68,908