Changes in Accumulated Other Comprehensive Income by Component - Reclassification Adjustments (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Tax (expense) benefit	$ (5,372)			$ (6,896)			$ (13,539)	$ (12,931)
Net income	18,344	$ 18,186	$ 9,156	26,086	$ 25,814	$ 72	45,686	51,972
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]
Gains (losses) from the sale of available-for-sale securities	14			(24)			117	(180)
Tax (expense) benefit	(3)			5			(27)	35
Net income	$ 11			$ (19)			$ 90	$ (145)