Acquisition and Discontinued Operations (Narrative) (Details) shares in Millions		3 Months Ended		12 Months Ended
	Jan. 01, 2020 USD ($) branch shares	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Business Acquisition [Line Items]
Goodwill		$ 146,800,000		$ 369,900,000
Interest income		513,600,000	$ 516,500,000
Non-interest income		340,400,000	314,500,000
Net loss		624,300,000	(118,900,000)
Loss from discontinued operations			300,000
Discontinued Operations
Business Acquisition [Line Items]
Loss from discontinued operations		0	300,000	$ 0
Net cash flows used in operations			5,100,000
Net cash provided by investing			$ 23,400,000
MOB
Business Acquisition [Line Items]
Consideration paid	$ 993,100,000
Business combination, percentage of outstanding shares of common stock	100.00%
Consideration paid in cash	$ 850,000,000
Assets acquired	8,600,000,000
Goodwill	121,600,000
Intangible assets	102,600,000
Assumed liabilities	$ 7,569,200,000
Number of bank branches acquired | branch	25
Cash and interest-bearing cash	$ 123,100,000
Investment securities	1,717,600,000
Fair value of loans	6,297,900,000
Other assets	199,500,000
Right of use assets	45,500,000
Equity investments	23,900,000
Fair value on derivative instruments	19,900,000
Accrued interest receivable on loans	16,000,000.0
Property, furniture and fixtures	14,900,000
Technology assets	12,500,000
Mortgage Servicing Rights	10,300,000
Other assets	56,600,000
Deposits	6,992,900,000
Certificate of deposits with maturity	1,200,000,000
Certificate of deposits without maturity	5,800,000,000
PAA premium	14,300,000
Securities sold under agreements to repurchase	193,200,000
Federal Home Loan Bank advances	290,000,000.0
Other liabilities	$ 93,100,000
Interest income		74,700,000
Non-interest income		14,100,000
Net loss		92,600,000
Provision for credit losses		44,800,000
Integration costs		$ 17,100,000
MOB | Minimum
Business Acquisition [Line Items]
Certificates of deposit, maturity term	30 days
MOB | Maximum
Business Acquisition [Line Items]
Certificates of deposit, maturity term	5 years
MOB | Common Stock
Business Acquisition [Line Items]
Business combination, number of shares issued | shares	3.1
Business combination, value of shares issued	$ 141,000,000