Summary of Significant Accounting Policies - Supplemental Cash Flow Information (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2018	Jan. 01, 2019
Cash Transactions:
Interest paid		$ 77,748	$ 68,403
Income taxes paid, net		620	550
Operating lease payments		3,424	3,381
Noncash Transactions:
Loans charged-off to the allowance for credit losses		11,693	6,068
Loans foreclosed upon and transferred to other real estate owned		1,800	624
Loans foreclosed upon and transferred to other assets		0	87
Available-for-sale securities transferred to held-to-maturity portfolio		873,613	0	$ 179,800
Right of use assets recognized during the period in exchange for lease obligations	[1]	716	$ 81,249
Recognized upon initial adoption of ASU		$ (31,796)
Accounting Standards Update 2016-02
Noncash Transactions:
Recognized upon initial adoption of ASU					$ 79,900

[1] ⁽¹⁾ Includes $79.9 million recognized upon initial adoption of ASU 2016-02 on January 1, 2019.