Short-Term Borrowings and Long-Term Debt - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Debt Disclosure [Line Items]
Interest expense on debt	$ 9.2	$ 9.0
Short-term debt, weighted average interest rate	0.84%		0.59%
Short-term FHLB advances
Debt Disclosure [Line Items]
Market value of collateral pledged	$ 1,700.0
Federal Reserve Bank Advances
Debt Disclosure [Line Items]
Market value of collateral pledged	$ 800.0