STATEMENTS OF CHANGES IN COMMON EQUITY AND COMPREHENSIVE INCOME - USD ($) $ in Thousands		Total		Common Stock	Other Paid-In Capital and Premium on Common Stock	Accumulated Other Comprehensive Loss	Retained Earnings
Beginning balance at Dec. 31, 2012		$ 521,395		$ 5,848	$ 201,050	$ (13,285)	$ 327,782
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income		62,236					62,236
Other Comprehensive Gain (Loss), Net of Tax	[2]	2,416	[1]			2,416
Cash Dividends Declared – Common Stock		0					0
Additional Investment by Shareholder		25,000			25,000
Tax Deficiency from Restricted Stock Plan		(78)			(78)
Ending balance at Dec. 31, 2013		$ 610,969		5,848	225,972	(10,869)	390,018
Additional Statement Information [Abstract]
Combined statutory rate		41.00%
Net Income		$ 66,483					66,483
Other Comprehensive Gain (Loss), Net of Tax	[2]	(3,610)	[1]			(3,610)
Cash Dividends Declared – Common Stock		(18,201)					(18,201)
Additional Investment by Shareholder		25,000			25,000
Tax Deficiency from Restricted Stock Plan		(73)			(73)
Ending balance at Dec. 31, 2014		$ 680,568		5,848	250,899	(14,479)	438,300
Additional Statement Information [Abstract]
Combined statutory rate		40.00%
Net Income		$ 66,578					66,578
Other Comprehensive Gain (Loss), Net of Tax	[2]	1,617	[1]			1,617
Cash Dividends Declared – Common Stock		(40,764)					(40,764)
Additional Investment by Shareholder		0			0
Tax Deficiency from Restricted Stock Plan		(72)			(72)
Ending balance at Dec. 31, 2015		$ 707,927		$ 5,848	$ 250,827	$ (12,862)	$ 464,114
Additional Statement Information [Abstract]
Combined statutory rate		40.00%

[1] Determined using a combined statutory tax rate of 40% in each of 2015 and 2014 and 41% in 2013.

[2] Determined using a combined statutory tax rate of 40% in each of 2015 and 2014 and 41% in 2013.