Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($)	Jun. 30, 2018		Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	$ 4,122,883,000	[1]	$ 3,531,427,000
Total equity securities	31,766,000
Total securities	4,154,649,000		3,531,427,000
Mortgage servicing rights	2,505,000		2,729,000
Loans held for sale	0		35,258,000
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	4,122,883,000		3,531,427,000
Total equity securities	31,766,000
Total securities	4,154,649,000
Mortgage servicing rights	2,505,000		2,729,000
Loans held for sale			35,258,000
Fair Value, Measurements, Recurring | Preferred stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale			15,434,000
Total equity securities	15,108,000
Fair Value, Measurements, Recurring | Mutual Funds and Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale			17,100,000
Total equity securities	16,658,000
Fair Value, Measurements, Recurring | GSE certificates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	1,923,416,000		2,068,842,000
Fair Value, Measurements, Recurring | GSE CMOs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	659,772,000		549,904,000
Fair Value, Measurements, Recurring | Mortgage-Related Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	2,583,188,000		2,618,746,000
Fair Value, Measurements, Recurring | U.S. Treasury obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	199,696,000		199,898,000
Fair Value, Measurements, Recurring | GSE debentures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	554,649,000		473,258,000
Fair Value, Measurements, Recurring | Asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	280,798,000
Fair Value, Measurements, Recurring | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	67,605,000		70,120,000
Fair Value, Measurements, Recurring | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	388,093,000		90,775,000
Fair Value, Measurements, Recurring | Capital trust notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	48,854,000		46,096,000
Fair Value, Measurements, Recurring | Other Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale			912,681,000
Fair Value, Measurements, Recurring | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	1,539,695,000
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	199,696,000		215,332,000
Total equity securities	15,108,000
Total securities	214,804,000
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Preferred stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale			15,434,000
Total equity securities	15,108,000
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	199,696,000		199,898,000
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale			215,332,000
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	199,696,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	3,923,187,000		3,316,095,000
Total equity securities	16,658,000
Total securities	3,939,845,000
Loans held for sale			35,258,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Mutual Funds and Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale			17,100,000
Total equity securities	16,658,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | GSE certificates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	1,923,416,000		2,068,842,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | GSE CMOs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	659,772,000		549,904,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-Related Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	2,583,188,000		2,618,746,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | GSE debentures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	554,649,000		473,258,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	280,798,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	67,605,000		70,120,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	388,093,000		90,775,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Capital trust notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	48,854,000		46,096,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Other Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale			697,349,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	1,339,999,000
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage servicing rights	$ 2,505,000		$ 2,729,000

[1] As equity securities have no contractual maturity, they have been excluded from this table.