Investments - Maturity Table (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,013,021	$ 485,756
Estimated Fair Value	2,020,948	$ 487,669
2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	416,977
Estimated Fair Value	417,051
2021
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	354,463
Estimated Fair Value	356,542
2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	814,700
Estimated Fair Value	816,138
2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	22,696
Estimated Fair Value	23,173
2038
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	173
Estimated Fair Value	196
2039
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,301
Estimated Fair Value	2,530
2042
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,922
Estimated Fair Value	7,016
2043
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,804
Estimated Fair Value	11,342
2044
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	14,552
Estimated Fair Value	15,093
2045
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	13,909
Estimated Fair Value	14,358
2046
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	22,838
Estimated Fair Value	23,480
2047
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	34,843
Estimated Fair Value	35,351
2048
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,824
Estimated Fair Value	9,190
2049
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	67,357
Estimated Fair Value	69,867
2050
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	145,958
Estimated Fair Value	145,735
2054
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	75,704
Estimated Fair Value	$ 73,886