Significant Accounting Policies Significant Accounting Policies - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accrued interest receivable and other assets	$ 319,968		$ 319,968		$ 340,018
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,112		136,112		136,069
Other long-term borrowings, net of unamortized issuance costs	99,960		99,960		99,870
Income Tax Expense (Benefit)	$ 10,852	$ 12,130	$ 28,622	$ 36,814
Earnings Per Share, Diluted	$ 1.24	$ 1.17	$ 3.42	$ 3.39
Excess Tax Benefit from Share-based Compensation, Operating Activities			$ 1,610	$ 880
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, net of unamortized issuance costs					1,000
Other long-term borrowings, net of unamortized issuance costs					$ 130
Income Tax Expense (Benefit)	$ (1,500)		$ (1,600)
Earnings Per Share, Diluted	$ 0.02		$ 0.03
Excess Tax Benefit from Share-based Compensation, Operating Activities				880
Excess Tax Benefit from Share-based Compensation, Financing Activities				$ (880)