Investments - Maturity Table (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 454,603	$ 182,325
Estimated Fair Value	455,968	$ 176,245
2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	77,265
Estimated Fair Value	77,377
2021
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	103,336
Estimated Fair Value	103,698
2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	34,997
Estimated Fair Value	34,917
2038
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	260
Estimated Fair Value	284
2039
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,880
Estimated Fair Value	3,071
2042
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,558
Estimated Fair Value	7,388
2043
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	12,274
Estimated Fair Value	12,420
2044
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	18,838
Estimated Fair Value	18,994
2045
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	20,227
Estimated Fair Value	20,259
2046
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	32,152
Estimated Fair Value	32,028
2047
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	48,145
Estimated Fair Value	47,816
2048
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	13,205
Estimated Fair Value	13,548
2049
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	83,466
Estimated Fair Value	$ 84,168