Changes in Accumulated Other Comprehensive Income by Component - Reclassification Adjustments (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Tax benefit									$ (16,386)	$ (44,739)	$ (42,991)
Net income	$ 31,552	$ 26,086	$ 25,814	$ 72	$ 20,186	$ 23,472	$ 22,551	$ 22,017	83,524	88,226	77,396
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 from the sale of available-for-sale securities									197	11	222
Tax benefit									(32)	(4)	(79)
Net income									$ 165	$ 7	$ 143