Incentive Plans - Changes in Stock Options Outstanding (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Number of Shares
Exercised (in shares)	(11,359)	(12,345)	(21,200)
Outstanding at end of year (in shares)	2,048,400
Exercisable at end of year (in shares)	2,048,400
Stock Options
Number of Shares
Outstanding at beginning of year (in shares)	2,920,410	3,994,595	4,901,269
Exercised (in shares)	(47,000)	(57,500)	(72,000)
Forfeited (in shares)	(825,010)	(1,016,685)	(834,674)
Outstanding at end of year (in shares)	2,048,400	2,920,410	3,994,595
Exercisable at end of year (in shares)	2,048,400	2,694,410	3,182,345	3,197,269
Average Exercise Price
Outstanding at beginning of year (in USD per share)	$ 43.93	$ 44.66	$ 45.74
Exercised (in USD per share)	17.57	17.57	17.57
Forfeited (in USD per share)	54.85	48.29	53.36
Outstanding at end of year (in USD per share)	40.14	43.93	44.66
Exercisable at end of year (in USD per share)	$ 37.88	$ 43.51	$ 49.10	$ 54.22