Changes in Accumulated Other Comprehensive Income by Component - Reclassification Adjustments Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Tax expense (benefit)	$ 9,458	$ 10,860	$ 30,247	$ 32,761
Net income	23,472	19,711	68,040	57,097
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	171	(17)	(2)	226
Tax expense (benefit)	(54)	8	1	(82)
Net income	$ 117	$ (9)	$ (1)	$ 144