Common Stock (Details) - USD ($)	3 Months Ended		222 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Dec. 31, 2019
Equity, Class of Treasury Stock [Line Items]
Common stock, outstanding	54,837,771		54,837,771	56,219,831
Common stock, issued	101,695	100,947
Common stock, dividend, per share	$ 0.08
Number of shares repurchased	1,500,000		17,400,000
Stock repurchased (in US dollars)	$ 99,048,000		$ 890,500,000
Remaining authorized repurchase amount (in US dollars)	259,500,000		259,500,000
RepurchaseProgramSep2011toMar2020
Equity, Class of Treasury Stock [Line Items]
Stock repurchase, authorized amount (in US dollars)	$ 1,150,000,000		$ 1,150,000,000