Changes in Accumulated Other Comprehensive Income by Component - Reclassification Adjustments (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Tax benefit (expense)	$ (9,488)			$ (5,372)			$ (23,391)	$ (13,539)
Net income	27,644	$ 23,101	$ 20,190	18,344	$ 18,186	$ 9,156	70,935	45,686
Realized Gains (Losses) on Sale of Available-for-sale Securities | Amounts Reclassified from Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
(Losses) gains from the sale of available-for-sale securities	(7)			14			(94)	117
Tax benefit (expense)	2			(3)			23	(27)
Net income	$ (5)			$ 11			$ (71)	$ 90