Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Investment securities:
Securities held to maturity, fair value (in dollars)	$ 1,915,461
Allowance for credit losses	$ 0
Shareholders' equity:
Common stock, par value (in dollars per share)	$ 2.50	$ 2.50	$ 2.50
Common stock, shares authorized	80,000,000	80,000,000	80,000,000
Common stock, shares issued	33,444,236	33,744,385	35,368,521
Common stock, shares outstanding	33,444,236	33,744,385	35,368,521