Debt (Details)	6 Months Ended	12 Months Ended
	Jun. 30, 2016 USD ($) financial_institution line_of_credit	Dec. 31, 2015 USD ($)
Short-term Debt [Line Items]
Short-term debt outstanding	$ 587,228,000	$ 581,700,000
Commercial Paper
Short-term Debt [Line Items]
Short-term debt outstanding	$ 537,200,000	$ 581,700,000
Weighted average interest rates on short-term debt outstanding	0.60%	0.50%
Average daily borrowings	$ 475,800,000	$ 387,900,000
Weighted average interest rates on average daily borrowings	0.60%	0.30%
Line of Credit | SBC
Short-term Debt [Line Items]
Number of uncommitted lines of credit | line_of_credit	3
Number of financial institutions offering lines of credit | financial_institution	4
Number of financial institutions with stated limits | line_of_credit	2
Incremental increase amount in principal allowed	$ 200,000,000
Number of financial institutions with no stated limits | line_of_credit	1
Uncommited Lines of Credit
Short-term Debt [Line Items]
Average daily borrowings	$ 6,300,000	$ 3,900,000
Weighted average interest rates on average daily borrowings	0.80%	1.20%
Uncommited Lines of Credit | SBC
Short-term Debt [Line Items]
Short-term debt outstanding	$ 50,000,000	$ 0
Weighted average interest rates on short-term debt outstanding	1.00%