Changes in Accumulated Other Comprehensive Income by Component - Summary (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated other comprehensive income
Beginning balance			$ 399,952
Other comprehensive (loss) income before reclassifications			(1,694)	$ 1,162
Losses reclassified from other comprehensive income			145	1
Net current period change in other comprehensive (loss) income			(1,549)	1,163
Ending balance	$ 419,580		419,580
Gains (losses) from the sale of available-for-sale securities	(100)	$ 200	(200)	(100)
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	$ (712)	$ 844	$ (712)	$ 844

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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