Disclosures About Fair Value of Financial Instruments - Estimated Fair Values of Financial Instruments (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Carrying Amount	$ 1,067,330	$ 212,505
Cash and cash equivalents, Estimated Fair Value	1,067,330	212,505
Debt securities, available for sale	664,758	577,198
Debt securities, available for sale, Estimated Fair Value	664,758	577,198
Interest-only classes of agency-guaranteed home equity conversion mortgage-backed securities, reported at fair value based on a fair value option election, Carrying Amount	14,396	16,272
Interest-only classes of agency-guaranteed home equity conversion mortgage-backed securities, reported at fair value based on a fair value option election, Estimated Fair Value	14,396	16,272
Equity securities, Carrying Amount	2,228	2,406
Equity securities, Estimated Fair Value	2,228	2,406
Loans held for sale	464,164	486,328
Total loans and leases receivable, net of allowance for credit losses on loans and leases	14,666,133	9,508,367
Total loans and leases receivable, net of allowance for credit losses on loans and leases, Estimated Fair Value	15,287,585	9,853,037
FHLB, Federal Reserve Bank and other restricted stock, Carrying Amount	91,023	84,214
FHLB, Federal Reserve Bank and other restricted stock, Estimated Fair Value	91,023	84,214
Derivative assets	65,127	23,608
Deposits, Carrying Amount	10,965,875	8,648,936
Deposits, Estimated Fair Value	10,972,163	8,652,340
FRB advances, Carrying Amount	4,419,967
FRB advances, Estimated Fair Value	4,419,967
Federal funds purchased, Carrying Amount	0	538,000
Federal funds purchased, Estimated Fair Value		538,000
FHLB advances, Carrying Amount	850,000	850,000
FHLB advances, Estimated Fair Value	855,519	852,162
Other borrowings, Carrying Amount	123,833	123,630
Other borrowings, Estimated Fair Value	100,200	127,603
Subordinated debt, Carrying Amount	181,255	181,115
Subordinated debt, Estimated Fair Value	174,572	192,217
Derivative liabilities	125,304	45,939
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Estimated Fair Value	1,067,330	212,505
Debt securities, available for sale, Estimated Fair Value	0	0
Interest-only classes of agency-guaranteed home equity conversion mortgage-backed securities, reported at fair value based on a fair value option election, Estimated Fair Value	0	0
Equity securities, Estimated Fair Value	2,228	2,406
Loans held for sale	0	0
Total loans and leases receivable, net of allowance for credit losses on loans and leases, Estimated Fair Value	0	0
FHLB, Federal Reserve Bank and other restricted stock, Estimated Fair Value	0	0
Derivative assets	0	0
Deposits, Estimated Fair Value	9,095,495	6,980,402
FRB advances, Estimated Fair Value	0
Federal funds purchased, Estimated Fair Value		538,000
FHLB advances, Estimated Fair Value	0	0
Other borrowings, Estimated Fair Value	0	0
Subordinated debt, Estimated Fair Value	0	0
Derivative liabilities	0	0
Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Estimated Fair Value	0	0
Debt securities, available for sale, Estimated Fair Value	664,758	577,198
Interest-only classes of agency-guaranteed home equity conversion mortgage-backed securities, reported at fair value based on a fair value option election, Estimated Fair Value	0	0
Equity securities, Estimated Fair Value	0	0
Loans held for sale	21,107	2,130
Total loans and leases receivable, net of allowance for credit losses on loans and leases, Estimated Fair Value	2,793,164	2,245,758
FHLB, Federal Reserve Bank and other restricted stock, Estimated Fair Value	91,023	84,214
Derivative assets	65,075	23,529
Deposits, Estimated Fair Value	1,876,668	1,671,938
FRB advances, Estimated Fair Value	4,419,967
Federal funds purchased, Estimated Fair Value		0
FHLB advances, Estimated Fair Value	855,519	852,162
Other borrowings, Estimated Fair Value	100,200	127,603
Subordinated debt, Estimated Fair Value	174,572	192,217
Derivative liabilities	125,304	45,939
Significant Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Estimated Fair Value	0	0
Debt securities, available for sale, Estimated Fair Value	0	0
Interest-only classes of agency-guaranteed home equity conversion mortgage-backed securities, reported at fair value based on a fair value option election, Estimated Fair Value	14,396	16,272
Equity securities, Estimated Fair Value	0	0
Loans held for sale	443,057	484,198
Total loans and leases receivable, net of allowance for credit losses on loans and leases, Estimated Fair Value	12,494,421	7,607,279
FHLB, Federal Reserve Bank and other restricted stock, Estimated Fair Value	0	0
Derivative assets	52	79
Deposits, Estimated Fair Value	0	0
FRB advances, Estimated Fair Value	0
Federal funds purchased, Estimated Fair Value		0
FHLB advances, Estimated Fair Value	0	0
Other borrowings, Estimated Fair Value	0	0
Subordinated debt, Estimated Fair Value	0	0
Derivative liabilities	$ 0	$ 0