Changes in Accumulated Other Comprehensive Income by Component - Reclassification Adjustments Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2017	Jun. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Tax expense (benefit)	$ (10,527)		$ (10,923)		$ (20,789)	$ (21,901)
Net income	22,551		18,760		44,568	37,386
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	(125)	$ (48)	181	$ 62	(173)	243
Tax expense (benefit)	40	15	(67)	(23)	55	(90)
Net income	$ (85)	$ (33)	$ 114	$ 39	$ (118)	$ 153