Fair Value - Fair Value Measurements on a Recurring Basis by Pricing Source (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair value by pricing source
Available-for-sale securities	$ 730,701
Total	797,834
Level 1
Fair value by pricing source
Total	24,233
Level 2
Fair value by pricing source
Total	761,956
Level 3
Fair value by pricing source
Total	11,645
Fair Value, Measurements, Recurring Basis
Fair value by pricing source
Available-for-sale securities	730,701
Total	797,834	$ 763,582
Fair Value, Measurements, Recurring Basis | Pricing services
Fair value by pricing source
Available-for-sale securities	730,551
Equity securities priced using pricing services	67,133
Fair Value, Measurements, Recurring Basis | Internal modeling
Fair value by pricing source
Available-for-sale securities	150
Fair Value, Measurements, Recurring Basis | Level 1
Fair value by pricing source
Available-for-sale securities	2,683
Total	24,233	1,809
Fair Value, Measurements, Recurring Basis | Level 1 | Pricing services
Fair value by pricing source
Available-for-sale securities	2,683
Equity securities priced using pricing services	21,550
Fair Value, Measurements, Recurring Basis | Level 1 | Internal modeling
Fair value by pricing source
Available-for-sale securities	0
Fair Value, Measurements, Recurring Basis | Level 2
Fair value by pricing source
Available-for-sale securities	716,373
Total	761,956	745,990
Fair Value, Measurements, Recurring Basis | Level 2 | Pricing services
Fair value by pricing source
Available-for-sale securities	716,373
Equity securities priced using pricing services	45,583
Fair Value, Measurements, Recurring Basis | Level 2 | Internal modeling
Fair value by pricing source
Available-for-sale securities	0
Fair Value, Measurements, Recurring Basis | Level 3
Fair value by pricing source
Available-for-sale securities	11,645
Total	11,645	$ 12,577
Fair Value, Measurements, Recurring Basis | Level 3 | Pricing services
Fair value by pricing source
Available-for-sale securities	11,495
Equity securities priced using pricing services	0
Fair Value, Measurements, Recurring Basis | Level 3 | Internal modeling
Fair value by pricing source
Available-for-sale securities	$ 150