Shareholders' Equity - Stock Buyback Program (Details) - USD ($) shares in Millions	3 Months Ended
	Mar. 31, 2018	Feb. 14, 2018
Equity, Class of Treasury Stock [Line Items]
Treasury stock, value acquired	$ 40,182,000
Common shares | February 2018 Stock Buyback Program
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, authorized amount		$ 200,000,000
Treasury stock, shares acquired (shares)	1.8
Treasury stock, value acquired	$ 40,200,000