Commitments and Contingencies - Low-Income Housing Commitments (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Commitments and Contingencies Disclosure [Abstract]
2018	$ 27,326
2019	9,198
2020	3,257
2021	2,213
2022	2,169
Thereafter	$ 246,836

Purchase Commitments Under Low Income Housing Program Due in Second Year

No definition available.

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Purchase Commitments Under Low Income Housing Program Due in Third Year

No definition available.

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Purchase Commitments Under Low Income Housing Program Due Within One Year

No definition available.

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Purchase Commitments Under Low Income Housing Program Due in Fifth Year

No definition available.

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Purchase Commitments Under Low Income Housing Program Due in Fourth Year

No definition available.

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Purchase Commitments Under Low Income Housing Program Thereafter

No definition available.

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Data Type:	xbrli:monetaryItemType
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No definition available.

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