Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2017	Dec. 31, 2018
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 21,029		$ 17,617
Short-term finance receivables allowance	13,633		12,454
Long-term finance receivables allowance	$ 8,518		$ 7,768
Preferred stock par value (in dollars per share)	$ 50		$ 50
Preferred stock dividend rate	4.00%	4.00%
Preference stock, par value (in dollars per share)	$ 0		0
Preference stock dividend rate (in dollars per share)	2.12		2.12
Common stock, par value (in dollars per share)	$ 1		$ 1
Common stock, shares authorized (in shares)	480,000,000		480,000,000
Common stock, shares issued (in shares)	323,337,912		323,337,912
Treasury stock (in shares)	140,812,458		135,662,830