Stockholder's Equity - Changes in the Number of Shares Outstanding and Treasury Shares Held by the Company (Details) - shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Shares Outstanding [Abstract]
Beginning balance	284,026,820	241,388,277
Common stock issued	2,740,096	2,586,578
Purchase of common stock	(867,203)	(933,060)
Ending balance	285,899,713	243,041,795
Shares Held as Treasury Shares [Abstract]
Beginning balance	8,499,021	8,302,004
Purchase of common stock	867,203	933,060
Retirement of common stock	(858,682)	(787,267)
Ending balance	8,507,542	8,447,797
Common stock, shares issued	294,407,255	251,489,592	292,525,841