STATEMENTS OF COMMON STOCKHOLDER'S EQUITY (USD $) In Thousands, except Share data, unless otherwise specified	Common Stock [Member]	Additional Paid In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Total
Balance at Dec. 31, 2009	$ 0	$ 692,948	$ 258,409	$ (1,847)	$ 949,510
Balance (in shares) at Dec. 31, 2009	100
Comprehensive income:
Net income			78,067		78,067
Other comprehensive income				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
Comprehensive income:
Net income			89,901		89,901
Other comprehensive income				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
Comprehensive income:
Net income			106,369		106,369
Other comprehensive income				172	172
Comprehensive income					106,541
Common dividends declared to parent			(66,469)		(66,469)
Contribution of capital by parent		60,024			60,024
Balance at Dec. 31, 2012	$ 0	$ 843,186	$ 335,101	$ (1,332)	$ 1,176,955
Balance (in shares) at Dec. 31, 2012	100				100