Debt (Details) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Short-term Debt [Line Items]
Short-term debt outstanding	$ 454,642,000	$ 488,988,000
Commercial Paper [Member]
Short-term Debt [Line Items]
Short-term debt outstanding	$ 404,600,000	$ 489,000,000
Weighted average interest rates on short-term debt outstanding	0.30%	0.30%
Average daily borrowings	$ 417,100,000	$ 335,000,000
Weighted average interest rates on average daily borrowings	0.30%	0.20%
Notes Payable to Banks [Member]
Short-term Debt [Line Items]
Short-term debt outstanding	$ 50,000,000	$ 0
Weighted average interest rates on short-term debt outstanding	1.40%
Average daily borrowings	$ 4,200,000	$ 5,500,000
Weighted average interest rates on average daily borrowings	1.20%	1.10%