Deposits (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deposits, by Type [Abstract]
Non-interest bearing demand	$ 3,158,300,000	$ 2,900,000,000
Interest bearing:
Demand	2,957,500,000	2,787,500,000
Savings	3,247,900,000	3,095,400,000
Total	547,600,000	432,000,000
Time, other	769,400,000	720,000,000
Total interest bearing	7,522,400,000	7,034,900,000
Total deposits	10,680,700,000	9,934,900,000
Brokered time deposits	24,100,000	0
Time deposits obtained through Certificate of Deposit Account Registry Service (CDARS)	87,100,000	94,200,000
Time deposits of $250,000 or more	221,000,000	182,000,000
FDIC deposit insurance limit	250,000,000
Time, $100 and Over
Due within 3 months or less	45,000,000
Due after 3 months and within 6 months	49,000,000
Due after 6 months and within 12 months	217,500,000
Due within 2020	159,700,000
Total	547,600,000	432,000,000
Total Time
Due within 3 months or less	253,800,000
Due after 3 months and within 6 months	149,200,000
Due after 6 months and within 12 months	420,400,000
Due within 2020	323,700,000
Total	1,317,000,000
Interest expense, time deposits, $100,000 or more	7,000,000	$ 4,300,000	$ 3,700,000
Contractual Maturities, Time Deposits, $100,000 or More, after 2 years	63,500,000
Time Deposits Maturities, after Next Two Years	128,800,000
Contractual Maturities, Time Deposits, $100,000 or More, after 3 years	9,800,000
Time Deposits Maturities, after Next Three Years	30,300,000
Contractual Maturities, Time Deposits, $100,000 or More, after 5 years and thereafter	3,100,000
Time Deposits Maturities, after Next Five Years and Thereafter	$ 10,800,000