Income Tax (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	$ 24,608	$ 18,892
Income Tax Reconciliation, Differences Regulatory Commission	1,518	1,329
Income Tax Reconciliation, Other Adjustments	(740)	(1,138)
Income Tax Expense (Benefit)	$ 25,386	$ 19,083
Effective Income Tax Rate	40.90%	40.10%