Parent Company Condensed Financial Statements (Condensed Balance Sheet) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
ASSETS
Cash placed with subsidiary bank	$ 2,174,592	$ 1,878,503	$ 1,360,887	$ 1,039,885
Available-for-sale investment securities, at fair value	3,016,752	3,335,795
Investments in tax credit investments, net	224,551	173,280
Other assets	678,952	782,400
TOTAL	37,150,249	34,788,840	32,350,922
LIABILITIES
Long-term debt	171,577	186,327
Total liabilities	33,308,298	31,361,099
STOCKHOLDERS’ EQUITY
Common stock, $0.001 par value, 200,000,000 shares authorized; 165,214,770 and 164,604,072 shares issued in 2017 and 2016, respectively	165	164
Additional paid-in capital	1,755,330	1,727,434
Retained earnings	2,576,302	2,187,676
Treasury stock, at cost — 20,671,710 shares in 2017 and 20,436,621 shares in 2016	(452,327)	(439,387)
Accumulated other comprehensive loss, net of tax	(37,519)	(48,146)
Total stockholders’ equity	3,841,951	3,427,741	3,122,950	2,856,111
TOTAL	37,150,249	34,788,840
Parent Company
ASSETS
Cash placed with subsidiary bank	159,566	39,264	$ 18,898	$ 211,053
Available-for-sale investment securities, at fair value	0	9,338
Investments in tax credit investments, net	25,511	32,245
Other assets	7,062	4,812
TOTAL	4,026,499	3,638,318
LIABILITIES
Long-term debt	171,577	186,327
Other liabilities	12,971	24,250
Total liabilities	184,548	210,577
STOCKHOLDERS’ EQUITY
Common stock, $0.001 par value, 200,000,000 shares authorized; 165,214,770 and 164,604,072 shares issued in 2017 and 2016, respectively	165	164
Additional paid-in capital	1,755,330	1,727,434
Retained earnings	2,576,302	2,187,676
Treasury stock, at cost — 20,671,710 shares in 2017 and 20,436,621 shares in 2016	(452,327)	(439,387)
Accumulated other comprehensive loss, net of tax	(37,519)	(48,146)
Total stockholders’ equity	3,841,951	3,427,741
TOTAL	4,026,499	3,638,318
Parent Company | Bank
ASSETS
Investments in subsidiaries	3,830,696	3,546,984
Parent Company | Nonbank
ASSETS
Investments in subsidiaries	$ 3,664	$ 5,675