Net Income Per Share - Stock Repurchase Program (Details) - USD ($) shares in Millions, $ in Millions	Aug. 15, 2018	Feb. 20, 2018	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Stock repurchase program authorized amount			$ 235.0	$ 135.0
Stock repurchase program increase in authorized repurchase amount	$ 75.0	$ 25.0
Cumulative amount repurchased			$ 199.4
Cumulative number of shares repurchased			14.7