Note 6 - Stock-based Compensation - Summary of Restricted Common Stock Activity (Details) - Restricted Stock [Member] - $ / shares	3 Months Ended
Mar. 31, 2017	Mar. 31, 2016
Outstanding - beginning of period (in shares)	396,033	337,506
Outstanding - beginning of period (in dollars per share)	$ 12.06	$ 9.57
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	307,943	237,500
Granted (in dollars per share)	$ 10.40	$ 12.88
Vested (in shares)	(200,291)	(178,973)
Vested (in dollars per share)	$ 11.82	$ 8.46
Outstanding - end of period (in shares)	503,685	396,033
Outstanding - end of period (in dollars per share)	$ 11.14	$ 12.06
Common Class A [Member]
Outstanding - beginning of period (in shares)	415,082	374,693
Outstanding - beginning of period (in dollars per share)	$ 10.15	$ 9.46
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	198,220	218,612
Granted (in dollars per share)	$ 10.60	$ 11.25
Vested (in shares)	(227,526)	(178,223)
Vested (in dollars per share)	$ 10	$ 10.04
Outstanding - end of period (in shares)	385,776	415,082
Outstanding - end of period (in dollars per share)	$ 10.47	$ 10.15

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)-(ii)
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-SubTopic 10
-Section 50
-Paragraph 2
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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(2)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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-Publisher FASB
-Name Accounting Standards Codification
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-Subparagraph (c)(2)(iii)(2)
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