Significant Accounting Policies (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) ATM branch	Dec. 31, 2017 USD ($)	Jan. 01, 2018 USD ($)	Dec. 31, 2016 USD ($)
Accounting Policies [Abstract]
Number of branches | branch	250
Number of ATMs | ATM	328
Collateral requirements	$ 32,700	$ 43,800
Provisional income tax expense		47,200
Reclassification from AOCI to retained earnings, tax effect	7,600
Reclassification from AOCI to retained earnings, effect of regulatory capital	$ 7,600
Reclassification from AOCI to retained earnings, effect of regulatory capital, increase to Tier 1 capital	0.03%
Reclassification from AOCI to retained earnings, effect of regulatory capital, increase to common equity capital	0.03%
Reclassification from AOCI to retained earnings, effect of regulatory capital, increase to total risk based capital	0.03%
Reclassification from AOCI to retained earnings, effect of regulatory capital, increase to leverage ratio	0.02%
Basis Of Presentation [Line Items]
Retained earnings	$ 621,925	544,207
Adjustment from adoption of ASU		(685)		$ 0
ASU 2014-09
Basis Of Presentation [Line Items]
Adjustment from adoption of ASU		(685)
ASU 2016-01
Basis Of Presentation [Line Items]
Adjustment from adoption of ASU	3,200	0
ASU 2016-01 | Accumulated Other Comprehensive Income (Loss)
Basis Of Presentation [Line Items]
Adjustment from adoption of ASU		117
Difference between Revenue Guidance in Effect before and after Topic 606 | ASU 2014-09
Basis Of Presentation [Line Items]
Retained earnings			$ (685)
Cash and Cash Equivalents
Basis Of Presentation [Line Items]
Cash and cash equivalents	14,100	8,600
Interest earning deposits with banks	$ 6,300	$ 5,900