Summary of Significant Accounting Policies (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) Segment	Dec. 31, 2017 USD ($)
Description of Business [Abstract]
Number of reportable segments | Segment	1
Recently Adopted Accounting Pronouncements [Abstract]
Restricted cash	$ 4,024	$ 4,033
Corporate income tax rate	21.00%	35.00%
ASU 2016-01 [Member]
Recently Adopted Accounting Pronouncements [Abstract]
Cumulative-effect adjustment to reclassify unrealized gains on equity securities after tax	$ (46,157)
ASU 2016-18 [Member]
Recently Adopted Accounting Pronouncements [Abstract]
Restricted cash		$ 4,000
ASU 2018-02 [Member]
Recently Adopted Accounting Pronouncements [Abstract]
Cumulative-effect adjustment to reclassify unrealized gains on equity securities after tax	117
Other Comprehensive Income (Loss) [Member] | ASU 2016-01 [Member]
Recently Adopted Accounting Pronouncements [Abstract]
Cumulative-effect adjustment to reclassify unrealized gains on equity securities before tax		71,012
Cumulative-effect adjustment to reclassify unrealized gains on equity securities after tax		46,157
Other Comprehensive Income (Loss) [Member] | ASU 2018-02 [Member]
Recently Adopted Accounting Pronouncements [Abstract]
Cumulative-effect adjustment to reclassify unrealized gains on equity securities after tax	117
Retained Earnings [Member] | ASU 2016-01 [Member]
Recently Adopted Accounting Pronouncements [Abstract]
Cumulative-effect adjustment to reclassify unrealized gains on equity securities before tax		71,012
Cumulative-effect adjustment to reclassify unrealized gains on equity securities after tax		$ 46,157
Retained Earnings [Member] | ASU 2018-02 [Member]
Recently Adopted Accounting Pronouncements [Abstract]
Cumulative-effect adjustment to reclassify unrealized gains on equity securities after tax	$ 117