Derivative Instruments - Interest Rate Swap Agreements (Details) - Interest Rate Swap - USD ($)	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Notional Amount	$ 3,475,000,000	$ 3,600,000,000
Weighted-average: Fixed Pay Rate	2.10%	1.67%
Weighted-average: Variable Receive Rate	2.33%	1.52%
Weighted-average: Net Receive (Pay) Rate	0.23%	(0.15%)
Weighted-average: Remaining Life (in years)	7 years 1 month 6 days	5 years 1 month 6 days
Fair Value, Asset and Liability	$ (1,033,000)	$ (3,338,000)
Less Than Three Years Maturity
Notional Amount	$ 1,050,000,000	$ 1,300,000,000
Weighted-average: Fixed Pay Rate	1.53%	1.28%
Weighted-average: Variable Receive Rate	2.33%	1.51%
Weighted-average: Net Receive (Pay) Rate	0.80%	0.23%
Weighted-average: Remaining Life (in years)	1 year 9 months 18 days	1 year 9 months 18 days
Fair Value, Asset and Liability	$ (225,000)	$ (248,000)
Three To Less Than Seven Years Maturity
Notional Amount	$ 225,000,000	$ 700,000,000
Weighted-average: Fixed Pay Rate	1.95%	1.87%
Weighted-average: Variable Receive Rate	2.35%	1.48%
Weighted-average: Net Receive (Pay) Rate	0.40%	(0.39%)
Weighted-average: Remaining Life (in years)	3 years 6 months	3 years 10 months 24 days
Fair Value, Asset and Liability	$ (77,000)	$ (454,000)
Seven to Less Than Ten Years Maturity
Notional Amount	$ 2,050,000,000
Weighted-average: Fixed Pay Rate	2.34%
Weighted-average: Variable Receive Rate	2.33%
Weighted-average: Net Receive (Pay) Rate	(0.01%)
Weighted-average: Remaining Life (in years)	8 years 7 months 6 days
Fair Value, Asset and Liability	$ (822,000)
Ten or More Years Maturity
Notional Amount	$ 150,000,000
Weighted-average: Fixed Pay Rate	3.01%
Weighted-average: Variable Receive Rate	2.32%
Weighted-average: Net Receive (Pay) Rate	(0.69%)
Weighted-average: Remaining Life (in years)	29 years 9 months 18 days
Fair Value, Asset and Liability	$ 91,000
Seven To Ten Year Maturity
Notional Amount		$ 1,600,000,000
Weighted-average: Fixed Pay Rate		1.90%
Weighted-average: Variable Receive Rate		1.55%
Weighted-average: Net Receive (Pay) Rate		(0.35%)
Weighted-average: Remaining Life (in years)		8 years 3 months 18 days
Fair Value, Asset and Liability		$ (2,636,000)

Derivative average interest rate net.

No definition available.

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Namespace Prefix:	ai_
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Balance Type:	na
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- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

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Period Type:	instant

- Definition

Average fixed interest rate related to the group of interest rate derivatives.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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Period Type:	instant

- Definition

Average remaining period until maturity of the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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- Definition

Average variable interest rate related to the group of interest rate derivatives.

+ References

No definition available.

+ Details

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- Definition

Fair value of the assets less the liabilities of a derivative or group of derivatives.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611

+ Details

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- Details

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- Details

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