Short-Term Borrowings and Long-Term Debt - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Debt Disclosure [Line Items]
Interest expense on debt	$ 9.0	$ 9.7	$ 27.2	$ 28.2
Short-term debt, weighted average interest rate	106.00%		106.00%		0.59%
Short-term FHLB advances
Debt Disclosure [Line Items]
Market value of collateral pledged	$ 1,900.0		$ 1,900.0
Federal Reserve Bank Advances
Debt Disclosure [Line Items]
Market value of collateral pledged	$ 700.0		$ 700.0