Earnings per Common Share - Calculation of Basic and Diluted Weighted-Average Shares Outstanding (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Weighted-average shares - basic (in shares)	398,521	415,514	445,538
Dilution effect of stock awards at end of period (in shares)	2,001	1,937	2,030
Weighted-average shares - diluted (in shares)	400,522	417,451	447,568