Summary of Significant Accounting Policies (Narrative) (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash on hand or on deposit required	$ 176,600	$ 188,900
Cash collateral on deposit	14,700	16,900
Foreclosed Assets	2,400	2,300
Accrued interest receivable and other assets	396,654	340,018
Junior subordinated deferrable interest debentures	136,127	136,069
Other long-term borrowings, net of unamortized issuance costs	99,990	99,870
Income Tax Expense (Benefit)	$ 37,150	$ 40,471	$ 58,047
Earnings Per Share, Diluted	$ 4.70	$ 4.28	$ 4.29
Excess Tax Benefit from Share-based Compensation, Operating Activities	$ 5,063	$ 1,434	$ 3,202
Minimum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Range of change in fair value of derivative hedging instrument to be considered highly effective	80.00%
Maximum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Range of change in fair value of derivative hedging instrument to be considered highly effective	125.00%
Adjustments for New Accounting Pronouncement [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accrued interest receivable and other assets		(1,200)
Junior subordinated deferrable interest debentures		1,000
Other long-term borrowings, net of unamortized issuance costs		130
Income Tax Expense (Benefit)	$ 5,100
Earnings Per Share, Diluted	$ 0.08
Excess Tax Benefit from Share-based Compensation, Operating Activities		1,400	3,200
Excess Tax Benefit from Share-based Compensation, Financing Activities		$ (1,400)	$ (3,200)