Changes in Accumulated Other Comprehensive Income by Component - Reclassification Adjustments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Tax benefit	$ 7,502	$ 10,527	$ 6,035	$ 20,789
Net income	25,814	22,551	25,886	44,568
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	(67)	(125)	(156)	(173)
Tax benefit	11	40	30	55
Net income	$ (56)	$ (85)	$ (126)	$ (118)