Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Deferred policy acquisition costs		$ 288,578				$ 271,186				$ 288,578		$ 271,186		$ 252,612
Reserve for loss and loss expenses		4,260,355				4,067,163				4,260,355		4,067,163		3,893,868
Unearned premiums		1,618,271				1,523,167				1,618,271		1,523,167		1,431,932
Net premiums earned										2,681,814		2,597,171		2,436,229
Net investment income	[1]									222,890		236,965		140,413
Loss and loss expense incurred										1,635,823		1,551,491		1,498,134
Amortization of deferred policy acquisition costs										560,271		535,973		495,042
Other operating expenses										349,371	[2]	345,714	[3]	321,880	[4]
Net premiums written										2,773,092		2,679,424		2,514,286
Other insurance expenses										366,941		358,069		331,318
Other income		4,943	$ 6,119	$ 4,683	$ 1,825	3,820	$ 3,162	$ 3,053	$ 2,320	17,570		12,355		9,438
Standard Commercial Lines [Member]
Deferred policy acquisition costs		246,494				226,464				246,494		226,464		206,391
Reserve for loss and loss expenses		3,596,340				3,436,363				3,596,340		3,436,363		3,283,531
Unearned premiums		1,196,243				1,108,009				1,196,243		1,108,009		1,020,054
Net premiums earned										2,143,184		2,049,614		1,912,222
Net investment income										0		0		0
Loss and loss expense incurred										1,245,627		1,187,856		1,141,038
Amortization of deferred policy acquisition costs										474,322		445,661		412,420
Other operating expenses										271,504		270,107		249,660
Net premiums written										2,230,636		2,137,071		1,975,683
Other income										15,512		10,889		8,180
E&S Lines [Member]
Deferred policy acquisition costs		28,281				27,874				28,281		27,874		28,151
Reserve for loss and loss expenses		435,667				406,600				435,667		406,600		387,114
Unearned premiums		113,845				106,033				113,845		106,033		107,793
Net premiums earned										239,490		239,818		219,566
Net investment income										0		0		0
Loss and loss expense incurred										156,936		152,335		150,344
Amortization of deferred policy acquisition costs										55,255		55,835		49,005
Other operating expenses										27,173		21,905		20,912
Net premiums written										247,290		237,761		229,326
Other income										0		0		1
Miscellaneous Income [Member]
Other income										17,570		12,355		9,438
Standard Personal Lines
Deferred policy acquisition costs		13,803				16,848				13,803		16,848		18,070
Reserve for loss and loss expenses		228,348				224,200				228,348		224,200		223,223
Unearned premiums		308,183				309,125				308,183		309,125		304,085
Net premiums earned										299,140		307,739		304,441
Net investment income										0		0		0
Loss and loss expense incurred										233,260		211,300		206,752
Amortization of deferred policy acquisition costs										30,694		34,477		33,617
Other operating expenses										50,694		53,702		51,308
Net premiums written										295,166		304,592		309,277
Other income										2,058		1,466		1,257
Investments SIGI
Deferred policy acquisition costs		0				0				0		0		0
Reserve for loss and loss expenses		0				0				0		0		0
Unearned premiums		$ 0				$ 0				0		0		0
Net premiums earned										0		0		0
Net investment income	[1]									222,890		236,965		140,413
Loss and loss expense incurred										0		0		0
Amortization of deferred policy acquisition costs										0		0		0
Other operating expenses										0		0		0
Net premiums written										$ 0		$ 0		$ 0

[1] Includes “Net investment income earned” and “Net realized and unrealized investment (losses) gains” on the Consolidated Statements of Income.

[2] “Other operating expenses” of $349,371 reconciles to the Consolidated Statements of Income

[3] “Other operating expenses” of $345,714 reconciles to the Consolidated Statements of Income

[4] “Other operating expenses” of $321,880 reconciles to the Consolidated Statements of Income