Other Comprehensive Income (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss at beginning of period		$ 811,850
Accumulated other comprehensive loss at end of period		876,589	$ 811,850
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Adoption of ASU No. 2018-02		0
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax		(123,588)	(112,037)	$ (118,874)
Income tax expense (benefit)		44,172	42,902	44,238
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss at beginning of period		(133)	(209)
Accumulated other comprehensive loss at end of period		(69)	(133)	$ (209)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
(Gains) losses reclassified from net accumulated other comprehensive loss		76	76
Net current period other comprehensive income (loss)		76	76
Adoption of ASU No. 2018-02	[1]	(12)
Gains and Losses on Cash Flow Hedges | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax		126	127
Income tax expense (benefit)		(50)	(51)
Total, net of tax		76	76
Gains and Losses on Cash Flow Hedges | Interest Rate Derivatives | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	[2]	$ 126	$ 127

[1] In 2017, NSP-Wisconsin implemented ASU No. 2018-02 related to the TCJA, which resulted in reclassification of certain credit balances within net accumulated other comprehensive loss to retained earnings. For further information, see Note 2.

[2] Included in interest charges.