Commercial Mortgage and Subordinate Loans, Net - Allocation of Carrying Value of Loan Portfolio Based on Internal Risk Ratings (Details) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2018 USD ($) loan	Dec. 31, 2017 USD ($) loan
Financing Receivable, Recorded Investment [Line Items]
Number of loans \| loan	70	59
Carrying value \| $	$ 4,866,735	$ 3,679,758
% of Portfolio	100.00%	100.00%
1 [Member]
Financing Receivable, Recorded Investment [Line Items]
Number of loans \| loan	0	0
Carrying value \| $	$ 0	$ 0
% of Portfolio	0.00%	0.00%
2 [Member]
Financing Receivable, Recorded Investment [Line Items]
Number of loans \| loan	4	5
Carrying value \| $	$ 263,216	$ 399,326
% of Portfolio	5.00%	10.00%
3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Number of loans \| loan	63	51
Carrying value \| $	$ 4,367,766	$ 3,034,358
% of Portfolio	90.00%	83.00%
4 [Member]
Financing Receivable, Recorded Investment [Line Items]
Number of loans \| loan	1	1
Carrying value \| $	$ 170,685	$ 168,208
% of Portfolio	4.00%	5.00%
5 [Member]
Financing Receivable, Recorded Investment [Line Items]
Number of loans \| loan	2	2
Carrying value \| $	$ 65,068	$ 77,866
% of Portfolio	1.00%	2.00%

Mortgage Loans on Real Estate, Commercial and Consumer, Percentage of Portfolio

No definition available.

Name:	ari_MortgageLoansOnRealEstateCommercialAndConsumerPercentageOfPortfolio
Namespace Prefix:	ari_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of mortgage loans within investment in mortgage loan on real estate by entity with substantial portion of business acquiring and holding investment real estate or interest in real estate.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 12
-Footnote 3
-Subsection 29

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 948
-SubTopic 310
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-29(Footnote 3))
-URI http://asc.fasb.org/extlink&oid=6589523&loc=d3e617274-123014

+ Details

Name:	srt_MortgageLoansOnRealEstateNumberOfLoans
Namespace Prefix:	srt_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The balance represents the amount of loans that are secured by real estate mortgages, offset by the reserve to cover probable credit losses on the loan portfolio.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(c))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

Name:	us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=ari_RiskLevelVeryLowMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_RiskLevelLowMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=ari_RiskLevelModerateAverageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_RiskLevelHighMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=ari_RiskLevelImpairedLossLikelyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: