Schedule III - Supplementary Insurance Information (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Deferred policy acquisition costs	$ 185,608		$ 172,981		$ 155,523
Reserve for loss and loss expenses	3,477,870		3,349,770		4,068,941
Unearned premiums	1,095,819		1,059,155		974,706
Net premiums earned	1,852,609		1,736,072		1,584,119
Net investment income	165,307	[1]	155,375	[1]	140,865	[1]
Losses and loss expenses incurred	1,157,501		1,121,738		1,120,990
Amortization of deferred policy acquisition costs	364,295	[2]	331,828	[3]	298,547	[4]
Other operating expenses	252,670	[5]	243,740	[6]	228,589	[7]
Net premiums written	1,885,280		1,810,159		1,666,883
Policy acquisition costs	624,470		579,977		526,143
Other income	(16,945)		(12,294)		(9,118)
Other expenses	33,673		35,686		30,462
Total	616,965		575,568		527,136
Standard Commercial Lines [Member]
Deferred policy acquisition costs	147,285		138,397		123,861
Reserve for loss and loss expenses	3,000,796		2,877,087		2,753,556
Unearned premiums	734,697		708,861		642,032
Net premiums earned	1,415,712		1,316,619		1,225,335
Net investment income	0	[1]	0	[1]	0	[1]
Losses and loss expenses incurred	870,018		831,261		853,143
Amortization of deferred policy acquisition costs	295,774	[2]	270,443	[3]	247,016	[4]
Other operating expenses	188,699	[5]	181,059	[6]	166,111	[7]
Net premiums written	1,441,047		1,380,740		1,263,738
Standard Personal Lines [Member]
Deferred policy acquisition costs	17,495		18,149		17,690
Reserve for loss and loss expenses	279,761		312,411		1,195,082
Unearned premiums	285,777		286,969		275,886
Net premiums earned	296,747		294,332		279,555
Net investment income	0	[1]	0	[1]	0	[1]
Losses and loss expenses incurred	197,182		206,450		204,644
Amortization of deferred policy acquisition costs	34,851	[2]	33,097	[3]	33,684	[4]
Other operating expenses	48,178	[5]	46,140	[6]	44,741	[7]
Net premiums written	292,061		297,757		289,848
Excess And Surplus Operations [Member]
Deferred policy acquisition costs	20,828		16,435		13,972
Reserve for loss and loss expenses	197,313		160,272		120,303
Unearned premiums	75,345		63,325		56,788
Net premiums earned	140,150		125,121		79,229
Net investment income	0	[1]	0	[1]	0	[1]
Losses and loss expenses incurred	90,301		84,027		63,203
Amortization of deferred policy acquisition costs	33,670	[2]	28,288	[3]	17,847	[4]
Other operating expenses	15,793	[5]	16,541	[6]	17,737	[7]
Net premiums written	152,172		131,662		113,297
Investments Segment [Member]
Deferred policy acquisition costs	0		0		0
Reserve for loss and loss expenses	0		0		0
Unearned premiums	0		0		0
Net premiums earned	0		0		0
Net investment income	165,307	[1]	155,375	[1]	140,865	[1]
Losses and loss expenses incurred	0		0		0
Amortization of deferred policy acquisition costs	0	[2]	0	[3]	0	[4]
Other operating expenses	0	[5]	0	[6]	0	[7]
Net premiums written	0		0		0
Miscellaneous Income [Member]
Other income	(16,598)	[5]	(12,201)	[6]	(8,827)	[7]
Other Income [Member]
Other income	(347)		(93)		(291)
Miscellaneous Expense [Member]
Other expenses	9,093	[5]	7,792	[6]	9,820	[7]
Other Expense [Member]
Other expenses	$ 24,580		$ 27,894		$ 20,642

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

[2] The total of “Amortization of deferred policy acquisition costs” of $364,295 and “Other operating expenses” of $252,670 reconciles to the Consolidated Statements of Income as follows:

[3] The total of “Amortization of deferred policy acquisition costs” of $331,828 and “Other operating expenses” of $243,740 reconciles to the Consolidated Statements of Income as follows:

[4] The total of “Amortization of deferred policy acquisition costs” of $298,547 and “Other operating expenses” of $228,589 reconciles to the Consolidated Statements of Income as follows:

[5] In addition to amounts related to the Standard Commercial Lines, Standard Personal Lines, and E&S Lines, “Other income” and “Other expenses” on the Consolidated Statements of Income includes holding company income and expense amounts of $347 and $24,580, respectively.

[6] In addition to amounts related to the Standard Commercial Lines, Standard Personal Lines, and E&S Lines, “Other income” and “Other expenses” on the Consolidated Statements of Income includes holding company income and expense amounts of $93 and $27,894, respectively.

[7] In addition to amounts related to the Standard Commercial Lines, Standard Personal Lines, and E&S Lines, “Other income” and “Other expenses” on the Consolidated Statements of Income includes holding company income and expense amounts of $291 and $20,642, respectively.