Stock-Based Compensation - Restricted Stock Activity (Details) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Shares (in 000’s)
Non-vested at beginning of period (in shares)	153	225	266
Granted (in shares)	133	77	138
Vested (in shares)	(119)	(146)	(178)
Forfeited (in shares)	(5)	(3)	(1)
Non-vested at end of period (in shares)	162	153	225
Weighted-Average Grant Date Fair Value
Non-vested at beginning of the year (in dollars per share)	$ 57.43	$ 51.31	$ 39.28
Granted (in dollars per share)	64.14	63.30	53.64
Vested (in dollars per share)	59.67	51.10	35.09
Forfeited (in dollars per share)	60.11	60.17	63.19
Non-vested at end of the year (in dollars per share)	$ 61.19	$ 57.43	$ 51.31