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	Postretirement Liability Adjustment		Unrealized Gain (Loss) on Available-for-Sale Securities		Unrealized Gain (Loss) on Derivatives
Beginning balance at Dec. 31, 2013		$ 610,969		$ 5,848	$ 225,972	$ (10,869)		$ 390,018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income		66,483						66,483
Other Comprehensive Gain (Loss), Net of Tax		(3,610)	[1],[2]			(3,610)	[2],[3]		$ (3,165)	[3]	$ (472)	[3]	$ 27	[3]
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%
Beginning balance at Dec. 31, 2013	[3]					(10,869)			(10,672)		397		(594)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Changes During Year		$ (3,610)	[1],[2]			(3,610)	[2],[3]		(3,165)	[3]	(472)	[3]	27	[3]
Ending balance at Dec. 31, 2014	[3]					(14,479)			(13,837)		(75)		(567)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income		66,578						66,578
Other Comprehensive Gain (Loss), Net of Tax		1,617	[1],[2]			1,617	[2],[3]		1,617	[3]	(23)	[3]	23	[3]
Cash Dividends Declared – Common Stock		(40,764)						(40,764)
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%
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Changes During Year		$ 1,617	[1],[2]			1,617	[2],[3]		1,617	[3]	(23)	[3]	23	[3]
Ending balance at Dec. 31, 2015		(12,862)				(12,862)	[3]		(12,220)	[3]	(98)	[3]	(544)	[3]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income		69,045						69,045
Other Comprehensive Gain (Loss), Net of Tax		(2,072)	[1],[2]			(2,072)	[2],[3]		(2,197)	[3]	98	[3]	27	[3]
Additional Investment by Shareholder		65,000			65,000
Ending balance at Dec. 31, 2016		$ 839,900		$ 5,848	$ 315,827	(14,934)		$ 533,159
Additional Statement Information [Abstract]
Combined statutory rate		40.00%
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Changes During Year		$ (2,072)	[1],[2]			(2,072)	[2],[3]		(2,197)	[3]	98	[3]	27	[3]
Ending balance at Dec. 31, 2016		$ (14,934)				$ (14,934)	[3]		$ (14,417)	[3]	$ 0	[3]	$ (517)	[3]

[1] Determined using a combined statutory tax rate of 40% in each of 2016, 2015 and 2014.

[2] Determined using a combined statutory tax rate of 40%.

[3] Determined using a combined statutory tax rate of 40%.