Merger - Summary of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands		3 Months Ended
	Aug. 01, 2019	Dec. 31, 2019	Dec. 31, 2018
Purchase price consideration:
Stock	$ 3,018,556
Fair value of assets acquired:
Cash and cash equivalents	975,014
Federal Home Loan Bank and Federal Reserve Bank stocks	218,582
Investment securities	3,774,738
Loans held-for-sale	44,532
Loans and leases	15,730,605
Premises and equipment	140,219
Loan servicing rights	59,567
Other intangible assets	159,532
Net deferred tax asset	61,647
Other assets	552,432
Total assets acquired	21,716,868
Fair value of liabilities assumed:
Deposits	16,418,215
Short-term borrowings	2,629,426
Long-term borrowings	442,323
Other liabilities	353,469
Total liabilities assumed	19,843,433
Fair value of net identifiable assets	1,873,435
Goodwill	$ 1,145,121	$ 1,299,878	$ 154,757
Loans and leases, adjustment		4,400
Intangible asset, adjustment		(42,000)
Net deferred tax asset, adjustment		12,300
Other assets, adjustment		(1,400)
Deposits, adjustment		196
Other liabilities, adjustment		7,900
Goodwill, adjustment		$ 34,800