Borrowed Funds - Additional Information (Detail) - USD ($)			1 Months Ended		3 Months Ended								12 Months Ended
		Sep. 27, 2013	Jul. 31, 2015	Jun. 30, 2015	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Weighted average interest rate for short-term borrowings					0.69%				0.28%				0.69%	0.28%
Prepaid long-term FHLB advances					$ 625,000,000				$ 150,000,000				$ 625,000,000	$ 150,000,000
Prepaid long-term FHLB stock					220,000,000				$ 125,000,000				220,000,000	$ 125,000,000
Contractual amounts due 2017					795,000,000								795,000,000
Contractual amounts due 2017					$ 50,000,000								$ 50,000,000
Combined weighted average interest rate					3.72%				4.52%				3.72%	4.52%
Loss on extinguishment of debt					$ (51,129,000)	$ 0	$ 0	$ 0	$ 10,132,000	$ 0	$ 0	$ 0	$ 51,129,000	$ 10,132,000	$ 0
Interest expense recorded on FHLB advances													67,200,000	78,200,000	78,400,000
Callable FHLB advances with scheduled maturities beyond 2017					$ 225,000,000								$ 225,000,000
Callable FHLB advances interest rates range, low					2.27%								2.27%
Callable FHLB advances interest rates range, high					4.18%								4.18%
Net carrying value of subordinated debentures					$ 130,600,000				131,300,000				$ 130,600,000	131,300,000
Interest expense on subordinated debentures													8,700,000	8,500,000	5,900,000
Interest expense on securities sold under agreements													17,100,000	24,200,000	$ 24,300,000
Fair value of securities					$ 1,400,000,000				$ 1,300,000,000				$ 1,400,000,000	$ 1,300,000,000
Subordinated debentures [Member]
Debt Instrument [Line Items]
Subordinated notes, interest rate			5.00%
Extinguishment of Debt, Amount			$ 100,000,000
Securities Sold under Agreements [Member]
Debt Instrument [Line Items]
Weighted average interest rate for long-term borrowings					3.35%				3.97%				3.35%	3.97%
Subordinated notes [Member]
Debt Instrument [Line Items]
Subordinated notes				$ 100,000,000			$ 100,000,000
Subordinated notes, interest rate				4.55%
Subordinated notes, due date													Jul. 30, 2025
FHLB [Member]
Debt Instrument [Line Items]
Weighted average interest rate for long-term borrowings					3.96%				3.83%				3.96%	3.83%
Collateralized letter of credit					$ 350,000,000								$ 350,000,000
Long-term borrowings [Member]
Debt Instrument [Line Items]
Contractual amounts due 2017					97,960,000								97,960,000
Contractual amounts due 2017					354,995,000								354,995,000
Subordinated notes	[1]				$ 230,573,000				$ 229,944,000				$ 230,573,000	$ 229,944,000
Subordinated debentures [Member]
Debt Instrument [Line Items]
Subordinated notes		$ 125,000,000
Subordinated notes, interest rate		5.125%
Subordinated notes, due date													Sep. 27, 2023

[1] Subordinated debt is presented net of debt issuance costs totaling $2.2 million and $1.4 million at December 31, 2015 and 2014, respectively.