Composition of Loan Portfolio (Detail) - USD ($) $ in Thousands		Dec. 31, 2017	Jul. 28, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered Loans		$ 38,359,022		$ 37,356,201
Net deferred loan origination costs		28,949		26,521
Allowance for losses on non-covered loans		(158,046)		(158,290)	$ (147,124)
Non-covered loans held for investment, net		38,229,925		37,224,432
Covered loans		0	$ 0	1,698,133
Allowance for losses on covered loans		0		(23,701)	(31,395)
Covered loans, net				1,674,432
Loans held for sale		35,258		409,152
Total loans, net		$ 38,265,183		$ 39,308,016
Non-Covered Loans, Percentage		100.00%		100.00%
Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered Loans	[1]	$ 2,040,574		$ 1,900,445
Commercial and Industrial | Other loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered Loans		$ 1,377,964		$ 1,341,216
Non-Covered Loans, Percentage		3.59%		3.59%
Multi-Family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered Loans		$ 28,074,709		$ 26,945,052
Non-Covered Loans, Percentage		73.19%		72.13%
Commercial Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered Loans		$ 7,322,226		$ 7,724,362
Non-Covered Loans, Percentage		19.09%		20.68%
One-to-four family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered Loans		$ 477,228		$ 381,081
Non-Covered Loans, Percentage		1.24%		1.02%
Acquisition, Development and Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered Loans		$ 435,825		$ 381,194
Non-Covered Loans, Percentage		1.14%		1.02%
Mortgage Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered Loans		$ 36,309,988		$ 35,431,689
Allowance for losses on non-covered loans		$ (128,275)		$ (125,416)	$ (124,478)
Non-Covered Loans, Percentage		94.66%		94.85%
Lease financing, unearned income
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered Loans		$ 662,610		$ 559,229
Non-Covered Loans, Percentage		1.73%		1.50%
Other Commercial and Industrial Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered Loans	[2]	$ 2,040,574		$ 1,900,445
Non-Covered Loans, Percentage	[2]	5.32%		5.09%
Purchased Credit-Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered Loans				$ 5,762
Non-Covered Loans, Percentage				0.01%
Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered Loans		$ 8,460		$ 18,305
Non-Covered Loans, Percentage		0.02%		0.05%
Total Other Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered Loans		$ 2,049,034		$ 1,918,750
Total Other Loans | Other loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered Loans		$ 2,049,034		$ 1,924,512
Non-Covered Loans, Percentage		5.34%		5.15%

[1] (1) Includes lease financing receivables, all of which were classified as "pass."

[2] Includes specialty finance loans of $1.5 billion and $1.3 billion, and other C&I loans of $500.8 million and $632.9 million, respectively, at December 31, 2017 and 2016.