Deposits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deposits, by Type [Abstract]
Non-interest bearing demand	$ 4,633,500	$ 3,426,500
Interest bearing:
Demand	4,118,900	3,195,400
Savings	4,405,900	3,591,600
Total	419,300	651,100
Time, other	639,400	798,900
Total interest bearing	9,583,500	8,237,000
Total deposits	14,217,000	11,663,500
Brokered time deposits	0	2,900
Time deposits obtained through Certificate of Deposit Account Registry Service (CDARS)	97,300	117,700
Time deposits of $250,000 or more	193,000	278,400
FDIC deposit insurance limit	250,000
Time, $100 and Over
Due within 3 months or less	65,800
Due after 3 months and within 6 months	77,200
Due after 6 months and within 12 months	173,000
Due within 2022	81,700
Due within 2023	7,300
Due within 2024	4,000
Due within 2025 and thereafter	10,300
Total	419,300	651,100
Total Time
Due within 3 months or less	271,100
Due after 3 months and within 6 months	190,800
Due after 6 months and within 12 months	355,500
Due within 2022	171,100
Due within 2023	30,000
Due within 2024	15,200
Due within 2025 and thereafter	25,000
Total	1,058,700
Interest expense, time deposits, $100,000 or more	$ 6,500	$ 11,500	$ 5,000