Stock-Based Compensation - Summary of Deferred Stock Awards to Non-Employee Directors (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Deferred awards outstanding, beginning of year (in shares)	65,679	132,699	189,099
Shares diversified (in shares)	0	0	0
Shares issued upon retirement (in shares)		(67,020)	(56,400)
Deferred awards outstanding, end of year (in shares)	65,679	65,679	132,699
Diversified Deferred Stock Award
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Deferred awards outstanding, beginning of year (in shares)	5,598	12,250	29,500
Shares diversified (in shares)	1,400	13,348	6,105
Shares issued upon retirement (in shares)		(20,000)	(23,355)
Deferred awards outstanding, end of year (in shares)	6,998	5,598	12,250
Undiversified Deferred Stock Award
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Deferred awards outstanding, beginning of year (in shares)	60,081	120,449	159,599
Shares diversified (in shares)	(1,400)	(13,348)	(6,105)
Shares issued upon retirement (in shares)		(47,020)	(33,045)
Deferred awards outstanding, end of year (in shares)	58,681	60,081	120,449