Condensed Statements of Financial Condition (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Assets
Cash on hand and in banks	$ 122,834	$ 109,995
Interest bearing cash deposits	318,550	35,135
Cash and cash equivalents	442,409	155,657
Investment securities, available-for-sale	2,387,428	3,222,829
Other assets	67,828	55,251
Total assets	8,306,507	7,884,350
Liabilities and Stockholders' Equity
Subordinated debentures	125,705	125,562
Other liabilities	102,026	50,103
Total liabilities	7,278,460	6,921,100
Common stock	750	744
Paid-in capital	708,356	690,918
Retained earnings	301,197	261,943
Accumulated other comprehensive income	17,744	9,645	47,962	33,487
Total stockholders’ equity	1,028,047	963,250	900,949	850,227
Total liabilities and stockholders’ equity	8,306,507	7,884,350
Parent Company
Assets
Cash on hand and in banks	4,019	1,582
Interest bearing cash deposits	51,127	49,097
Cash and cash equivalents	55,146	50,679
Investment securities, available-for-sale	91	87
Other assets	8,511	9,050
Investment in subsidiaries	1,121,937	1,040,104
Total assets	1,185,685	1,099,920
Liabilities and Stockholders' Equity
Dividends Payable	22,557	0
Subordinated debentures	125,705	125,562
Other liabilities	9,376	11,108
Total liabilities	157,638	136,670
Common stock	750	744
Paid-in capital	708,356	690,918
Retained earnings	301,197	261,943
Accumulated other comprehensive income	17,744	9,645
Total stockholders’ equity	1,028,047	963,250
Total liabilities and stockholders’ equity	$ 1,185,685	$ 1,099,920