Condensed Financial Statements of Parent Company (Schedule of Condensed Balance Sheets) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Cash	$ 545,542	$ 561,838
Resell agreements	159,967	18,742
Cash and cash equivalents	5,053,047	4,141,445	$ 3,591,523	$ 4,364,123
Total assets	31,747,880	30,196,319
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,184	136,127
Subordinated notes, net of unamortized issuance costs	98,552	99,990
Accrued interest payable and other liabilities	195,068	169,107
Total liabilities	28,450,017	27,193,791
Shareholders’ Equity	3,297,863	3,002,528	$ 2,890,343	$ 2,851,403
Total liabilities and shareholders’ equity	31,747,880	30,196,319
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Cash	9,301	9,854
Resell agreements	256,000	279,270
Cash and cash equivalents	265,301	289,124
Investment in subsidiaries	3,274,921	2,949,346
Accrued interest receivable and other assets	3,006	5,799
Total assets	3,543,228	3,244,269
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,184	136,127
Subordinated notes, net of unamortized issuance costs	98,552	99,990
Accrued interest payable and other liabilities	10,629	5,624
Total liabilities	245,365	241,741
Shareholders’ Equity	3,297,863	3,002,528
Total liabilities and shareholders’ equity	$ 3,543,228	$ 3,244,269