Stockholders' Equity (Schedule of Stock Options) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Number of Option Shares
Exercised (in shares)	(395,274)	(505,312)	(476,047)
Equity Plan
Number of Option Shares
Balance at beginning of the period (in shares)	212,030	453,331	723,661
Exercised (in shares)	(183,600)	(214,601)	(258,530)
Forfeited (in shares)	(1,550)	(26,700)	(11,800)
Reduction for cash dividend (in shares)		0	0
Balance at end of the period (in shares)	26,880	212,030	453,331
Exercisable at December 31, 2016 (in shares)	26,880
Weighted Average Exercise Price
Balance at beginning of the period (in usd per share)	$ 15.38	$ 16.89	$ 15.46
Exercised (in usd per share)	15.10	14.48	12.89
Forfeited (in usd per share)	17.17	14.30	16.93
Reduction for cash dividend (in usd per share)		(2.00)	(2.00)
Balance at end of the period (in usd per share)	17.17	$ 15.38	$ 16.89
Exercisable at December 31, 2016 (in usd per share)	$ 17.17