Condensed Financial Statements of Parent Company (Schedule of Condensed Balance Sheets) (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Cash	$ 678,791	$ 545,542
Resell agreements	635,017	159,967
Cash and cash equivalents	3,955,779	5,053,047	$ 4,141,445	$ 3,591,523
Total assets	32,292,966	31,747,880
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,242	136,184
Subordinated notes, net of unamortized issuance costs	98,708	98,552
Accrued interest payable and other liabilities	172,347	195,068
Total liabilities	28,924,049	28,450,017
Shareholders’ Equity	3,368,917	3,297,863	$ 3,002,528	$ 2,890,343
Total liabilities and shareholders’ equity	32,292,966	31,747,880
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Cash	11,397	9,301
Resell agreements	225,000	256,000
Cash and cash equivalents	236,397	265,301
Investment in subsidiaries	3,362,474	3,274,921
Accrued interest receivable and other assets	9,122	3,006
Total assets	3,607,993	3,543,228
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,242	136,184
Subordinated notes, net of unamortized issuance costs	98,708	98,552
Accrued interest payable and other liabilities	4,126	10,629
Total liabilities	239,076	245,365
Shareholders’ Equity	3,368,917	3,297,863
Total liabilities and shareholders’ equity	$ 3,607,993	$ 3,543,228