Deposits (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deposits, by Type [Abstract]
Non-interest bearing demand	$ 3,426,500,000	$ 3,158,300,000
Interest bearing:
Demand	3,195,400,000	2,957,500,000
Savings	3,591,600,000	3,247,900,000
Total	651,100,000	547,600,000
Time, other	798,900,000	769,400,000
Total interest bearing	8,237,000,000.0	7,522,400,000	$ 7,522,400,000	$ 7,034,900,000
Total deposits	11,663,500,000	10,680,700,000
Brokered time deposits	2,900,000	24,100,000	0
Time deposits obtained through Certificate of Deposit Account Registry Service (CDARS)	117,700,000	87,100,000
Time deposits of $250,000 or more	278,400,000	221,000,000.0
FDIC deposit insurance limit	250,000,000
Time, $100 and Over
Due within 3 months or less	101,600,000
Due after 3 months and within 6 months	140,800,000
Due after 6 months and within 12 months	273,700,000
Due within 2021	104,000,000.0
Total	651,100,000	547,600,000
Total Time
Due within 3 months or less	323,600,000
Due after 3 months and within 6 months	301,400,000
Due after 6 months and within 12 months	515,900,000
Due within 2021	220,400,000
Total	1,450,000,000.0
Interest expense, time deposits, $100,000 or more	11,500,000	$ 5,000,000.0	$ 3,500,000
Contractual Maturities, Time Deposits, $100,000 or More, after 2 years	23,600,000
Time Deposits Maturities, after Next Two Years	64,000,000.0
Contractual Maturities, Time Deposits, $100,000 or More, after 3 years	3,500,000
Time Deposits Maturities, after Next Three Years	13,900,000
Contractual Maturities, Time Deposits, $100,000 or More, after 5 years and thereafter	3,900,000
Time Deposits Maturities, after Next Five Years and Thereafter	$ 10,800,000