Deposits (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deposits, by Type [Abstract]
Non-interest bearing demand	$ 2,900,042,000	$ 1,906,257,000
Interest bearing:
Demand	2,787,500,000	2,276,500,000
Savings	3,095,400,000	2,141,800,000
Time, $100 and over	432,000,000	461,400,000
Time, other	720,000,000	590,100,000
Total interest bearing	7,034,829,000	5,469,853,000
Total deposits	9,934,871,000	7,376,110,000
Brokered time deposits	0	0
Time deposits obtained through Certificate of Deposit Account Registry Service (CDARS)	94,200,000	25,700,000
Time deposits of $250,000 or more	182,000,000	208,500,000
FDIC deposit insurance limit	250,000
Time, $100 and Over
Due within 3 months or less	88,600,000
Due after 3 months and within 6 months	65,300,000
Due after 6 months and within 12 months	110,600,000
Due after 12 months	167,500,000
Total	432,000,000	461,400,000
Total Time
Due within 3 months or less	313,800,000
Due after 3 months and within 6 months	167,300,000
Due after 6 months and within 12 months	259,200,000
Due after 12 months	411,600,000
Total	1,151,900,000
Interest expense, time deposits, $100,000 or more	$ 4,300,000	$ 3,700,000	$ 3,700,000