STATEMENTS OF COMMON STOCKHOLDER'S EQUITY AND COMPREHENSIVE INCOME (USD $) In Thousands, except Share data	Common Stock [Member]	Additional Paid In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Total
Balance at Dec. 31, 2008	$ 0	$ 676,705	$ 259,159	$ (5,559)	$ 930,305
Balance (in shares) at Dec. 31, 2008	100
Net income			67,750		67,750
Net derivative instrument changes during the period, net of tax				3,712	3,712
Comprehensive income					71,462
Common dividends declared to parent			(68,500)		(68,500)
Contribution of capital by parent		16,243			16,243
Balance at Dec. 31, 2009	0	692,948	258,409	(1,847)	949,510
Balance (in shares) at Dec. 31, 2009	100
Net income			78,067		78,067
Net derivative instrument changes during the period, net of tax				172	172
Comprehensive income					78,239
Common dividends declared to parent			(66,219)		(66,219)
Contribution of capital by parent		583			583
Balance at Dec. 31, 2010	0	693,531	270,257	(1,675)	962,113
Balance (in shares) at Dec. 31, 2010	100				100
Net income			89,901		89,901
Net derivative instrument changes during the period, net of tax				171	171
Comprehensive income					90,072
Common dividends declared to parent			(64,957)		(64,957)
Contribution of capital by parent		89,631			89,631
Balance at Dec. 31, 2011	$ 0	$ 783,162	$ 295,201	$ (1,504)	$ 1,076,859
Balance (in shares) at Dec. 31, 2011	100				100