Income Taxes (Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred income tax assets
Allowance for loan losses	$ 66,034	$ 100,419
Net operating loss carryforwards	41,059	167,072
Tax credit carryforwards	19,175	67,031
Employee benefits and deferred compensation	18,333	26,183
Net unrealized losses on investment securities available for sale	13,253	19,413
Deferred revenue	12,311	18,639
Non-performing loan interest	10,388	19,137
Other	8,892	14,600
Total gross deferred tax assets	189,445	432,494
Less valuation allowance	(3,431)	(9,658)
Total deferred tax assets	186,014	422,836
Deferred tax liabilities
Other properties held for sale	(5,447)	(8,179)
Excess tax over financial statement depreciation	(6,628)	(5,343)
Other	(8,151)	(13,958)
Total gross deferred tax liabilities	(20,226)	(27,480)
Net deferred tax asset	$ 165,788	$ 395,356