Stockholders' Equity (Schedule of Stock Awards) (Details) - Stock Awards - $ / shares	3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 31, 2015	Sep. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Shares
Balance at beginning of the period (in shares)	272,818	272,818	232,622	262,456	272,818
Granted (in shares)	96,010		103,382	86,918
Dividend equivalents (in shares)	8,992		5,249	3,811
Forfeited (in shares)	(16,534)			(10,832)
Exercised (in shares)	(98,830)		69,434	(109,731)
Balance at end of the period (in shares)			271,819	232,622	262,456
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Outstanding exercise price (in usd per share)	$ 39.03	$ 39.03	$ 55.60	$ 49.36	$ 39.03
Granted (in usd per share)			$ 90.36	$ 54.47	$ 60.29
Dividend equivalents (in usd per share)			104.93	88.04	59.94
Forfeited (in usd per share)		$ 60.29		$ 53.95
Exercised (in usd per share)			51.20	39.94	29.70
Outstanding exercise price (in usd per share)			$ 70.29	$ 55.60	$ 49.36