Other Assets - Changes in Amortized MSRs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in amortized mortgage servicing rights
Balance at beginning of year	$ 16,809	$ 21,435
Originations	27	65
Amortization	(3,640)	(4,691)	$ (5,500)
Balance at end of year	13,196	16,809	21,435
Fair value of amortized MSRs at end of year	21,697	25,160	29,676
Balance of loans serviced for others	2,342,619	2,702,489
Impairment of MSRs recorded	$ 0	$ 0	$ 0