Summary of Significant Accounting Policies - Effect of applying the guidance in ASU 2015-17 retrospectively to consolidated balance sheet (Details 7) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other Assets	$ 33,707	$ 32,784
Unamortized Debt Expense
Total Assets	1,868,907	1,818,683
Current Liabilities
Current Maturities of Long-Term Debt	85,490	52,422
Total Current Liabilities	246,256	271,116
Capitalization
Long-Term Debt - Net	460,757	443,846
Total Capitalization	1,117,476	1,048,869
Total Liabilities and Equity	$ 1,868,907	1,818,683
Previously Stated
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other Assets		31,108
Unamortized Debt Expense		3,897
Total Assets		1,820,904
Current Liabilities
Current Maturities of Long-Term Debt		52,544
Total Current Liabilities		271,238
Capitalization
Long-Term Debt - Net		445,945
Total Capitalization		1,050,968
Total Liabilities and Equity		1,820,904
Adjustments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other Assets		1,676
Unamortized Debt Expense		(3,897)
Total Assets		(2,221)
Current Liabilities
Current Maturities of Long-Term Debt		(122)
Total Current Liabilities		(122)
Capitalization
Long-Term Debt - Net		(2,099)
Total Capitalization		(2,099)
Total Liabilities and Equity		$ (2,221)