Non-Covered Loans and Allowance for Non-Covered Loan Losses (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Loans
Total loans	$ 4,834,687,000	$ 3,920,476,000
Allowance for loan losses	(39,365,000)	(37,041,000)
Non-covered loans, net	4,795,322,000	3,883,435,000
Noncovered
Loans
Total loans	4,834,687,000	3,920,476,000
Allowance for loan losses	(39,365,000)	(37,041,000)	(34,645,000)	(33,241,000)
Non-covered loans, net	4,795,322,000	3,883,435,000
Noncovered | Commercial and industrial
Loans
Total loans	2,221,727,000	1,758,851,000
Allowance for loan losses	(20,643,000)	(18,999,000)	(16,726,000)	(16,865,000)
Amount of margin loans to customers and correspondents included in the portfolio segment	608,600,000	378,400,000
Noncovered | Real estate
Loans
Total loans	2,067,259,000	1,694,835,000
Allowance for loan losses	(12,704,000)	(11,131,000)	(9,682,000)	(8,331,000)
Noncovered | Construction and land development
Loans
Total loans	493,293,000	413,643,000
Allowance for loan losses	(5,842,000)	(6,450,000)	(8,096,000)	(7,957,000)
Noncovered | Consumer
Loans
Total loans	52,408,000	53,147,000
Allowance for loan losses	$ (176,000)	$ (461,000)	$ (141,000)	$ (88,000)