Stockholders' Equity (Schedule of Deferred Stock Units Transactions) (Details) - Deferred Stock Unit - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Shares
Balance at beginning of the period (in shares)	738,288	506,447	527,513	738,288
Issued (in shares)	54,982	68,340	10,742
Granted (in shares)	90,184	95,079	173,097
Dividend equivalents (in shares)	20,922	9,799	9,075
Forfeited (in shares)	(23,604)	(3,094)	(10,893)
Exercised (in shares)	(353,259)	(227,516)	(203,087)
Balance at end of the period (in shares)		449,055	506,447	527,513
Weighted Average Price
Outstanding exercise price (in usd per share)	$ 36.96	$ 50.00	$ 44.94	$ 36.96
Issued (in usd per share)		108.61	72.01	47.51
Granted (in usd per share)		109.50	54.67	60.22
Dividend equivalent (in usd per share)		104.12	87.01	59.94
Forfeited (in usd per share)		72.96	48.98	44.78
Exercised (in usd per share)		40.39	43.55	32.62
Outstanding exercise price (in usd per share)		$ 72.55	$ 50.00	$ 44.94