Business Combinations - Summary of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands		3 Months Ended
	Aug. 01, 2019	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
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	15,713,399
Premises and equipment	140,219
Loan servicing rights	59,567
Other intangible assets	159,532
Net deferred tax asset	65,685
Other assets	552,432
Total assets acquired	21,703,700
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	1,860,267
Goodwill	$ 1,158,289		$ 1,313,046	$ 1,299,878
Adjustments to fair value, decrease in loans and leases		$ (17,200)
Adjustments to fair value, increase in deferred tax asset		4,000
Adjustments to fair value, increase in goodwill		$ 13,200