Debt Securities - Amortized Cost and Estimated Fair Value (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Face Amount	$ 544,190	$ 188,643
Amortized cost	534,226	190,178
Gross Unrealized Gain	7,322	2,313
Gross Unrealized Loss	(1,713)	(1,043)
Securities available-for-sale, at estimated fair value	539,835ari_AvailableforsaleSecuritiesFairValue	191,448ari_AvailableforsaleSecuritiesFairValue
Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Face Amount	17,013	33,066
Amortized cost	17,783	34,232
Gross Unrealized Gain	0	0
Gross Unrealized Loss	(678)	(870)
Securities available-for-sale, at estimated fair value	17,105	33,362
Commercial Mortgage Backed Securities [Member] \| AAA Commercial Mortage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Face Amount		155,577
Amortized cost		155,946
Gross Unrealized Gain		2,313
Gross Unrealized Loss		(173)
Securities available-for-sale, at estimated fair value		158,086
Commercial Mortgage Backed Securities [Member] \| Estimate of Fair Value [Member]
Schedule of Available-for-sale Securities [Line Items]
Face Amount	527,177
Amortized cost	516,443
Gross Unrealized Gain	7,322
Gross Unrealized Loss	(1,035)
Securities available-for-sale, at estimated fair value	$ 522,730

Available-for-sale Debt Securities, Face Amount

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- Definition

Available-for-sale Securities, Fair Value

+ References

No definition available.

+ Details

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- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

+ Details

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- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

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- Definition

This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (aa)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

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