STATEMENTS OF COMMON STOCKHOLDER'S EQUITY - USD ($) $ in Thousands	Total	Common stock	Additional Paid In Capital	Retained Earnings	AOCI Attributable to Parent
Beginning Balance at Dec. 31, 2014	$ 1,560,472	$ 0	$ 1,165,463	$ 395,998	$ (989)
Beginning Balance (in shares) at Dec. 31, 2014		100
Comprehensive income:
Net income	127,263			127,263
Other comprehensive income	(292)				(292)
Common dividends declared to parent	(85,254)			(85,254)
Contribution of capital by parent	205,760		205,760
Ending Balance at Dec. 31, 2015	1,807,949	$ 0	1,371,223	438,007	(1,281)
Ending Balance (in shares) at Dec. 31, 2015		100
Comprehensive income:
Net income	152,157			152,157
Other comprehensive income	(9)				(9)
Common dividends declared to parent	(103,401)			(103,401)
Contribution of capital by parent	75,000		75,000
Ending Balance at Dec. 31, 2016	$ 1,931,696	$ 0	1,446,223	486,763	(1,290)
Ending Balance (in shares) at Dec. 31, 2016	100	100
Comprehensive income:
Net income	$ 159,213			159,213
Other comprehensive income	83				83
Common dividends declared to parent	(104,648)			(104,648)
Contribution of capital by parent	144,019		144,019
Adoption of ASU No. 2018-02	0			260	(260)	[1]
Ending Balance at Dec. 31, 2017	$ 2,130,363	$ 0	$ 1,590,242	$ 541,588	$ (1,467)
Ending Balance (in shares) at Dec. 31, 2017	100	100

[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.