Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
AOCI Attributable to Parent, Net of Tax
Beginning balance	$ 790,244
Holding (losses) gains arising during period, net of tax	(454)	$ 92
Ending balance	739,995
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax
Beginning balance	(3,908)	(3,838)
Ending balance	$ (4,362)	$ (3,746)