Summary of Significant Accounting Policies - Investment Tax Credits, NMTC Fund, and Accounting for Renewable Energy Tax Credits (Details)	12 Months Ended
Dec. 31, 2018
Accounting Policies [Abstract]
Gross investment amortization expense period	10 years

The period of recognition (in years) of gross investment amortization expense using the cost method.

No definition available.

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Namespace Prefix:	cnl_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration