Fair Value Measurements - Fair Value by Balance Sheet Grouping (Details) - Fair Value Measurements Using Level 3 [Member] - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Carrying Amount [Member]
Liabilities
Carrying value of debt	$ 2,116,887	$ 2,068,867
Carrying Amount [Member] | LCIF [Member]
Liabilities
Carrying value of debt	402,890	370,581
Fair Value [Member]
Liabilities
Fair value of debt	2,061,819	2,013,226
Fair Value [Member] | LCIF [Member]
Liabilities
Fair value of debt	$ 385,292	$ 352,806