Investment in Securities, Securities Sold, Not Yet Purchased (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2017	Dec. 31, 2017	Sep. 30, 2018
Investment in Securities [Abstract]
Investment sold, not yet purchased	$ 0	$ 0	$ 0
Restricted investments [Abstract]
Restricted investments in securities held in escrow account	59,954	0	$ 0
New Accounting Pronouncement [Abstract]
Reclassification out of accumulated comprehensive income and into retained earnings, net of tax	$ 1,811	$ 12,100

Amount after tax of reclassification adjustments of other comprehensive income (loss).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 17B
-URI http://asc.fasb.org/extlink&oid=114867106&loc=SL34724394-108580

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
-URI http://asc.fasb.org/extlink&oid=114867106&loc=SL7669686-108580

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 55
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=109228884&loc=d3e1436-108581

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration