Common Stock (Schedule of Stock Option Activity) (Details) shares in Thousands	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Shares
Outstanding at beginning of period (in shares) | shares	507
Granted (in shares) | shares	105
Exercised (shares) | shares	(63)
Canceled (in shares) | shares	0
Outstanding at end of period (in shares) | shares	549
Exercisable (in shares) | shares	480
Weighted-Average Exercise Price
Outstanding at beginning of period (in dollars per share) | $ / shares	$ 21.37
Granted (in dollars per share) | $ / shares	60.89
Exercised (in dollars per share) | $ / shares	39.72
Canceled (in dollars per share) | $ / shares	0.00
Outstanding at end of period (in dollars per share) | $ / shares	26.80
Exercisable (in dollars per share) | $ / shares	$ 22.04