Changes in Accumulated Other Comprehensive Income by Component - Reclassification Adjustments (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Tax (benefit) expense									$ (44,739)	$ (42,991)	$ (31,740)
Net income	$ 20,186	$ 23,472	$ 22,551	$ 22,017	$ 20,299	$ 19,711	$ 18,760	$ 18,626	88,226	77,396	58,024
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 (gains) from the sale of available-for-sale securities									11	222	(27)
Tax (benefit) expense									(4)	(79)	10
Net income									$ 7	$ 143	$ (17)