FAIR VALUE MEASUREMENTS - Assets measured at fair value on nonrecurring basis (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Assets measured at fair value on a nonrecurring basis
Mortgage servicing rights	$ 334,500	$ 322,900
Non-recurring
Assets measured at fair value on a nonrecurring basis
Impaired loans	14,362	14,091
Other real estate owned	239	575
Non-recurring | Level III
Assets measured at fair value on a nonrecurring basis
Impaired loans	14,362	14,091
Other real estate owned	$ 239	$ 575