Fair Value Measurements - Level 3 Reconciliation (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at the beginning of the period	$ 3,600,000	$ 7,000,000	$ 3,500,000
Realized and unrealized gains (losses)	(200,000)	1,300,000	0
Purchases	15,200,000	3,900,000	15,600,000
Sales	(200,000)	(100,000)	0
Settlements	(13,300,000)	(11,900,000)	(12,100,000)
Acquisition of Integrys	0	(1,300,000)	0
Transfers out of level 3	0	4,700,000	0
Balance at the end of period	5,100,000	3,600,000	7,000,000
Unrealizes gains and losses on level 3 derivatives included in earnings	$ 0	$ 0	$ 0