Borrowings - Schedule of Borrowings (Schedule of Debt, Narrative) (Details)		1 Months Ended	12 Months Ended
	Dec. 06, 2010	Dec. 31, 2011	Dec. 31, 2017	Dec. 31, 2016	Dec. 09, 2011
4.2% Senior Notes, due December 2021
Debt Instrument [Line Items]
Stated interest rate			4.20%		4.20%
Senior Notes, maturity date		Dec. 15, 2021	Dec. 31, 2021	Dec. 31, 2021
4.5% Senior Notes, due December 2020
Debt Instrument [Line Items]
Stated interest rate	4.50%		4.50%
Senior Notes, maturity date	Dec. 15, 2020		Dec. 31, 2020	Dec. 31, 2020