Schedule II - Condensed Financial Information of Registrant - Condensed Statements of Cash Flow (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided (used) by operating activities	$ 148,996	$ 169,120	$ 111,124
(Investments in) distributions from unconsolidated subsidiaries, net:
Other partnership investments	(50,362)	(18,613)	(33,366)
Proceeds from sales or maturities of:
Fixed maturities, available for sale	932,070	752,516	886,886
Net decrease (increase) in short-term investments	4,167	(322,872)	11,932
Unsettled security transactions, net of change	(2,031)	1,388	2,339
Other	(9,380)	4,792	(2,505)
Net cash provided (used) by investing activities	224,667	(278,939)	227,787
Financing Activities
Borrowings (repayments) under Revolving Credit Agreement	(77,000)	100,000	100,000
Repurchase of common stock	0	(2,106)	(172,772)
Dividends to shareholders	(315,228)	(118,812)	(217,626)
Other	(7,683)	(2,968)	(5,289)
Net cash provided (used) by financing activities	(356,515)	(13,934)	(294,851)
Increase (decrease) in cash and cash equivalents	17,148	(123,753)	44,060
Cash and cash equivalents at beginning of period	117,347	241,100	197,040
Cash and cash equivalents at end of period	134,495	117,347	241,100
Supplemental Disclosure of Cash Flow Information
Cash paid during the year for interest	15,892	14,732	13,996
Significant non-cash transactions:
Dividends declared and not yet paid	267,292	265,659	69,447
Parent Company
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided (used) by operating activities	(1,669)	(10,549)	(14,411)
(Investments in) distributions from unconsolidated subsidiaries, net:
Other partnership investments	0	(1,000)	0
Proceeds from sales or maturities of:
Fixed maturities, available for sale	295,035	100,240	200,245
Net decrease (increase) in short-term investments	11,811	(262,169)	26,074
Dividends from subsidiaries	169,142	122,030	107,870
Contribution of capital to subsidiaries	0	(1,983)	0
Unsettled security transactions, net of change	1,100	(1,100)	0
Funds (advanced) repaid for Lloyd's FAL deposit	(25,449)	0	(9,642)
Funds (advanced) repaid under Syndicate Credit Agreement	(6,883)	1,695	(3,083)
Funds (advanced) repaid under a business investment line of credit	(4,066)	(3,090)	0
Other	(2,276)	(2,805)	(289)
Net cash provided (used) by investing activities	438,414	(48,182)	321,175
Financing Activities
Borrowings (repayments) under Revolving Credit Agreement	(77,000)	100,000	100,000
Repurchase of common stock	0	(2,106)	(172,772)
Subsidiary payments for common shares and share-based compensation awarded to subsidiary employees	12,030	11,384	6,063
Dividends to shareholders	(315,228)	(118,812)	(217,626)
Other	(6,868)	(3,697)	(6,337)
Net cash provided (used) by financing activities	(387,066)	(13,231)	(290,672)
Increase (decrease) in cash and cash equivalents	49,679	(71,962)	16,092
Cash and cash equivalents at beginning of period	31,330	103,292	87,200
Cash and cash equivalents at end of period	81,009	31,330	103,292
Supplemental Disclosure of Cash Flow Information
Cash paid during the year for income taxes, net of refunds	17,193	(8,519)	47,004
Cash paid during the year for interest	15,892	14,732	13,996
Significant non-cash transactions:
Dividends declared and not yet paid	267,292	265,659	69,447
Securities transferred at fair value as dividends from subsidiaries	190,709	174,270	206,880
Non-cash capital contribution to subsidiaries	$ 0	$ 0	$ 87,719