Pension Plan and Employee Benefits - 401 (K) Plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Defined Contribution Plan Disclosure [Line Items]
401(k) Plan expense	$ 1,424	$ 1,386	$ 4,716	$ 4,259
Other Subsidiaries
Defined Contribution Plan Disclosure [Line Items]
401(k) Plan expense	$ 254	$ 234	$ 865	$ 710