Changes in Accumulated Other Comprehensive Income by Component - Summary (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated other comprehensive income
Beginning balance			$ 399,952
Other comprehensive (loss) income before reclassifications			(1,167)	$ 963
Losses reclassified from other comprehensive income			126	118
Net current period change in other comprehensive (loss) income			(1,041)	1,081
Ending balance	$ 408,134		408,134
Losses from the sale of available-for-sale securities	(100)	$ (100)	(156)	(200)
Changes in other comprehensive income (loss), tax (benefit) expense			(300)	600
Accumulated Other Comprehensive Income (Loss), net of taxes
Accumulated other comprehensive income
Beginning balance			837	(319)
Ending balance	$ (204)	$ 762	$ (204)	$ 762

Amount of realized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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