Short-Term Debt - Additional Information (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Short Term Debt [Line Items]
Short-term debt	$ 313,979		$ 630,307
Commercial paper, discount	21		193
Proceeds from short-term borrowings with maturities greater than three months	35,000	20,000
Short-term borrowings with maturities more than three months repaid	35,000		221,000
Commercial Paper
Short Term Debt [Line Items]
Short-term debt	$ 313,979		$ 630,307