Changes in Accumulated Other Comprehensive Income by Component - Reclassification Adjustments (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Tax (expense) benefit	$ (5,431)		$ (7,502)		$ (8,167)	$ (6,035)
Net income	18,186	$ 9,156	25,814	$ 72	27,342	25,886
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	99		(67)		103	(156)
Tax (expense) benefit	(23)		11		(24)	30
Net income	$ 76		$ (56)		$ 79	$ (126)