Stockholders' Equity Share Repurchases (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) shares
Equity [Abstract]
Stock repurchased during period (in shares) | shares	138,638
Stock repurchased during period, value	$ 10.0
Cumulative amount of shares repurchased (in shares) | shares	1,139,295
Cumulative payments for repurchase of common stock	$ 92.9
Remaining dollar value of shares that may be repurchased	$ 57.1