Other Comprehensive Income (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period		$ 1,931,696
(Gains) losses reclassified from net accumulated other comprehensive loss		83	$ 139
Adoption of ASU No. 2018-02		0
Accumulated other comprehensive loss at end of period		2,130,363	1,931,696
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period		(1,290)	(1,281)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax			(148)
(Gains) losses reclassified from net accumulated other comprehensive loss		83	139
Net current period other comprehensive income (loss)		83	(9)
Adoption of ASU No. 2018-02	[1]	(260)
Accumulated other comprehensive loss at end of period		(1,467)	(1,290)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period		(612)	(464)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax			(148)
(Gains) losses reclassified from net accumulated other comprehensive loss		44	0
Net current period other comprehensive income (loss)		44	(148)
Adoption of ASU No. 2018-02	[1]	(123)
Accumulated other comprehensive loss at end of period		(691)	(612)
Gains and Losses on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period		(678)	(817)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax			0
(Gains) losses reclassified from net accumulated other comprehensive loss		39	139
Net current period other comprehensive income (loss)		39	139
Adoption of ASU No. 2018-02	[1]	(137)
Accumulated other comprehensive loss at end of period		$ (776)	$ (678)

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