Employee Benefit Plans (Amounts Recognized in the Financial Statements) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
BancShares Plan
Amounts recognized in the consolidated balance sheets
Other assets	$ 0	$ 0
Other liabilities	(36,949)	(72,611)
Net asset (liability) recognized	(36,949)	(72,611)
Amount recognized in accumulated other comprehensive income
Net loss (gain)	125,745	119,766
Less prior service cost	137	347
Accumulated other comprehensive loss, excluding income taxes	125,882	120,113
Actuarial loss	12,998
Prior service cost	79
Total	13,077
Bancorporation Plan
Amounts recognized in the consolidated balance sheets
Other assets	0	0
Other liabilities	(889)	(4,747)
Net asset (liability) recognized	(889)	(4,747)
Amount recognized in accumulated other comprehensive income
Net loss (gain)	19,117	21,661
Less prior service cost	0	0
Accumulated other comprehensive loss, excluding income taxes	19,117	$ 21,661
Actuarial loss	329
Prior service cost	0
Total	$ 329