Income Taxes - Summary of Consolidated Federal Income Tax Return (Details) - USD ($)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of consolidated Federal income tax return
Current city income tax provision	$ 1,463,669	$ 1,321,675	$ 1,245,285
Current state income tax provision	310,726	642,598	335,897
Current federal income tax provision	24,749,832	24,234,050	22,874,571
Deferred federal income tax expense (benefit)	2,893,063	469,312	(3,547,088)
Provision for income taxes	$ 29,417,290	$ 26,667,635	$ 20,908,665

Amount of deferred federal income tax expense (benefit) pertaining to income (loss) from continuing operations.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 6.I.7)
-URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(h))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32639-109319

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration