Investments - Maturity Table (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 331,519	$ 182,325
Available-for-sale investments	331,541	$ 176,245
2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	39,484
Available-for-sale investments	39,545
2021
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	37,689
Available-for-sale investments	37,787
2038
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	262
Available-for-sale investments	282
2039
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,907
Available-for-sale investments	3,065
2042
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,021
Available-for-sale investments	7,800
2043
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	13,727
Available-for-sale investments	13,776
2044
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	20,078
Available-for-sale investments	20,090
2045
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	22,915
Available-for-sale investments	22,728
2046
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	35,609
Available-for-sale investments	35,195
2047
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	53,183
Available-for-sale investments	52,759
2048
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	15,159
Available-for-sale investments	15,511
2049
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	82,485
Available-for-sale investments	$ 83,003