Short-Term Borrowings (Details) - USD ($)			1 Months Ended		12 Months Ended
	Oct. 22, 2015	Apr. 23, 2015	Oct. 31, 2015	Apr. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Short-term Debt [Line Items]
Short-term borrowings					$ 310,700,000	$ 12,000,000
Debt interest rate					0.85%
Line of Credit [Member] | Loan Facility [Member]
Short-term Debt [Line Items]
Committed amount			$ 50,000,000.0
Line of Credit [Member] | Loan Facility [Member] | LIBOR [Member]
Short-term Debt [Line Items]
Basis spread on variable rate			3.75%
Line of Credit [Member] | Intraday Credit Facility [Member]
Short-term Debt [Line Items]
Committed amount				$ 500,000,000.0
Debt term				6 months
Extension term				6 months
Line of Credit [Member] | Amended Intraday Credit Facility [Member]
Short-term Debt [Line Items]
Committed amount	$ 300,000,000.0
Extension term	364 days
Line of Credit [Member] | Amended Intraday Credit Facility [Member] | Federal Funds Rate [Member]
Short-term Debt [Line Items]
Basis spread on variable rate		0.50%