Incentive Plans - Changes in Stock Options Outstanding (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of Shares
Exercised	(12,345)	(21,200)	0
Outstanding at end of year	2,920,410
Exercisable at end of year	2,694,410
Stock Options
Number of Shares
Outstanding at beginning of year	3,994,595	4,901,269	5,757,435
Granted at FMV	0	0	(603,000)
Exercised	(57,500)	(72,000)	0
Forfeited	(1,016,685)	(834,674)	(1,459,166)
Outstanding at end of year	2,920,410	3,994,595	4,901,269	5,757,435
Exercisable at end of year	2,694,410	3,182,345	3,197,269	3,830,535
Average Exercise Price
Outstanding at beginning of year	$ 44.66	$ 45.74	$ 48.46
Granted at FMV	0	0	28.51
Exercised	17.57	17.57	0
Forfeited	48.29	53.36	49.34
Outstanding at end of year	43.93	44.66	45.74	$ 48.46
Exercisable at end of year	$ 43.51	$ 49.10	$ 54.22	$ 53.80