Shareholders' Equity - Stock Buyback Program and Prospectus Supplement (Details) - USD ($)	6 Months Ended
	Jun. 30, 2018	Feb. 14, 2018	Nov. 09, 2017
Equity, Class of Treasury Stock [Line Items]
Treasury stock, value acquired	$ 70,446,000
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)	3,117,483
Treasury stock, value acquired	$ 70,400,000
Prospectus Supplement
Equity, Class of Treasury Stock [Line Items]
Maximum common stock value authorized under prospectus supplement			$ 150,000,000
Issuance of common stock under prospectus supplement (in shares)	0