Schedule II - Condensed financial information of registrant (Balance Sheet) (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
ASSETS
Fixed maturities, at fair value	$ 5,543,030	$ 5,510,331
Cash and cash equivalents	419,976	723,109	$ 550,401	$ 729,333
Accrued investment income	26,488	23,897
Other assets	134,282	126,782
Total assets	11,349,755	10,515,812
LIABILITIES
Accounts payable and accrued expenses	587,648	627,331
Senior notes payable	245,362	245,161
Debentures and senior notes payable	782,588	782,829
Total liabilities	5,817,486	5,610,461
SHAREHOLDERS’ EQUITY
Common shares (Issued: 2016—161,279,976; 2015—160,570,772; Outstanding: 2016—79,132,252; 2015—82,900,617)	28,224	28,100
Treasury shares (2016—82,147,724; 2015—77,670,155)	(14,376)	(13,592)
Additional paid-in capital	821,023	1,002,980
Accumulated other comprehensive loss	(23,216)	(12,569)
Retained earnings	2,876,636	2,634,056
Total shareholders' equity	4,004,268	3,793,637	$ 3,878,860
Total liabilities and shareholders’ equity	$ 11,349,755	$ 10,515,812
Condensed Financial Information Balance Sheet Parenthetical Data [Abstract]
Preferred Stock, Shares Issued	6,000	0
Preferred Stock, Shares Outstanding	6,000	0
Authorized Share Capital	571,428,571	571,428,571
Common shares, par value	$ 0.175	$ 0.175
Common Stock, Shares, Issued	161,279,976	160,570,772	155,554,224	154,488,497
Common Stock, Shares, Outstanding	79,132,252	82,900,617	83,869,845
Treasury Stock, Shares	82,147,724	77,670,155
Parent Company
ASSETS
Fixed maturities, at fair value	$ 92,197	$ 28,403
Cash and cash equivalents	11,780	25,306	$ 29,798	$ 20,385
Investment in subsidiaries on an equity basis	4,383,800	4,198,809
Balances due from subsidiaries	0	10,389
Accrued investment income	239	49
Other assets	1,143	813
Total assets	4,489,159	4,263,769
LIABILITIES
Accounts payable and accrued expenses	26,884	29,633
Balances due to subsidiaries	28,622	0
Senior notes payable	245,362	245,161
Debentures and senior notes payable	350,000	350,000
Total liabilities	650,868	624,794
SHAREHOLDERS’ EQUITY
Preferred Stock, Value, Issued	150,000	0
Common shares (Issued: 2016—161,279,976; 2015—160,570,772; Outstanding: 2016—79,132,252; 2015—82,900,617)	28,224	28,100
Treasury shares (2016—82,147,724; 2015—77,670,155)	(14,376)	(13,592)
Additional paid-in capital	821,023	1,002,980
Accumulated other comprehensive loss	(23,216)	(12,569)
Retained earnings	2,876,636	2,634,056
Total shareholders' equity	3,838,291	3,638,975
Total liabilities and shareholders’ equity	$ 4,489,159	$ 4,263,769