Summary of Significant Accounting Policies, Recently Adopted Accounting Pronouncements (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Recently Adopted/Issued Accounting Pronouncements [Abstract]
Cumulative-effect adjustment to reclassify unrealized gains on equity securities after tax	$ 363,082	$ 364,316	$ 356,082	$ 418,811
Allowance for credit losses, net of tax	1,035
Right-of-use asset	120
Lease liability	120
ASU 2018-11 [Member]
Recently Adopted/Issued Accounting Pronouncements [Abstract]
Right-of-use asset	120
Lease liability	120
ASU 2016-13 [Member]
Recently Adopted/Issued Accounting Pronouncements [Abstract]
Allowance for credit losses, before tax	16,500	15,000
Allowance for credit losses, net of tax	13,035	11,850
Other Comprehensive Income (Loss) [Member]
Recently Adopted/Issued Accounting Pronouncements [Abstract]
Cumulative-effect adjustment to reclassify unrealized gains on equity securities after tax	21,759	9,369	(7,347)	46,391
Retained Earnings [Member]
Recently Adopted/Issued Accounting Pronouncements [Abstract]
Cumulative-effect adjustment to reclassify unrealized gains on equity securities after tax	$ 286,143	300,988	$ 308,075	316,700
Cumulative Effect, Period of Adoption, Adjustment [Member] | ASU 2016-01 [Member]
Recently Adopted/Issued Accounting Pronouncements [Abstract]
Cumulative-effect adjustment to reclassify unrealized gains on equity securities after tax				0
Cumulative Effect, Period of Adoption, Adjustment [Member] | ASU 2016-13 [Member]
Recently Adopted/Issued Accounting Pronouncements [Abstract]
Cumulative-effect adjustment to reclassify unrealized gains on equity securities after tax		(12,281)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Other Comprehensive Income (Loss) [Member] | ASU 2016-01 [Member]
Recently Adopted/Issued 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)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Other Comprehensive Income (Loss) [Member] | ASU 2016-13 [Member]
Recently Adopted/Issued Accounting Pronouncements [Abstract]
Cumulative-effect adjustment to reclassify unrealized gains on equity securities after tax		0
Cumulative Effect, Period of Adoption, Adjustment [Member] | Retained Earnings [Member] | ASU 2016-01 [Member]
Recently Adopted/Issued 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
Cumulative Effect, Period of Adoption, Adjustment [Member] | Retained Earnings [Member] | ASU 2016-13 [Member]
Recently Adopted/Issued Accounting Pronouncements [Abstract]
Cumulative-effect adjustment to reclassify unrealized gains on equity securities before tax		(15,545)
Cumulative-effect adjustment to reclassify unrealized gains on equity securities after tax		$ (12,281)