CONSOLIDATED STATEMENT OF CHANGES IN STOCKHOLDER'S EQUITY - USD ($) $ in Thousands	Total	Common Stock	Capital Stock Expense	Retained Earnings	Accumulated Other Comprehensive Loss
Balances at December 31 at Dec. 31, 2014	$ 1,215,779	$ 1,116,539	$ (6,357)	$ 111,523	$ (5,926)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	127,794			127,794
Other Comprehensive Income, Net of Tax	1,362				1,362
Dividends Declared to Parent	(50,000)			(50,000)
Contribution from Parent	180,000	180,000
Balances at December 31 at Dec. 31, 2015	1,474,935	1,296,539	(6,357)	189,317	(4,564)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	124,438			124,438
Other Comprehensive Income, Net of Tax	9				9
Dividends Declared to Parent	(50,000)			(50,000)
Balances at December 31 at Dec. 31, 2016	1,559,035	1,296,539	(6,357)	273,408	(4,555)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adoption of ASU, Cumulative Effect Adjustment	9,653			9,653
Net Income	176,668			176,668
Other Comprehensive Income, Net of Tax	(1,671)				(1,671)
Dividends Declared to Parent	(70,000)			(70,000)
Balances at December 31 at Dec. 31, 2017	$ 1,664,032	$ 1,296,539	$ (6,357)	$ 380,076	$ (6,226)