Schedule VI - Supplementary Information Concerning Property-Casualty Insurance Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Supplemental Information for Property, Casualty Insurance Underwriters [Abstract]
Deferred policy acquisition costs	$ 537,890	$ 513,128	$ 488,525
Reserves for losses and loss expenses	11,197,195	10,669,150	10,369,701
Unearned premiums	3,283,300	3,137,133	3,026,732
Net premiums earned	6,293,348	6,040,609	5,744,418
Net investment income	564,163	512,645	600,885
Losses and loss expenses incurred:
Current year	3,826,620	3,653,561	3,495,825	[1]
Prior years	(29,904)	(46,713)	(75,764)
Loss reserve discount accretion	49,084	49,422	70,506
Amortization of deferred policy acquisition costs	1,155,954	1,102,492	1,053,397
Paid losses and loss expenses	3,454,174	3,257,015	3,115,227
Net premiums written	$ 6,423,913	$ 6,189,515	$ 5,996,947

[1] Claims occurring during the current year are net of loss reserve discounts of $18,929,000, $20,357,000 and $21,306,000 in 2016, 2015, and 2014 , respectively.