Income Tax (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Income Tax Disclosure [Abstract]
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	$ 26,600	$ 24,608
Income Tax Reconciliation, Differences Regulatory Commission	1,518	1,518
Income Tax Reconciliation, Other Adjustments	(1,195)	(740)
Income Tax Expense (Benefit)	$ 26,923	$ 25,386
Effective Income Tax Rate	40.00%	40.90%