Reinsurance (Reinsurance Financial Information) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reinsurance Disclosures [Abstract]
Written premiums, Direct	$ 6,973,216,000	$ 6,726,029,000	$ 6,647,600,000
Written premiums, Assumed	729,278,000	750,934,000	896,101,000
Written premiums, Ceded	(1,269,267,000)	(1,216,455,000)	(1,119,788,000)
Total net premiums written	6,433,227,000	6,260,508,000	6,423,913,000
Earned premiums, Direct	6,851,795,000	6,661,046,000	6,492,240,000
Earned premiums, Assumed	755,759,000	812,309,000	900,570,000
Earned premiums, Ceded	(1,236,049,000)	(1,161,936,000)	(1,099,462,000)
Net premiums earned	6,371,505,000	6,311,419,000	6,293,348,000
Ceded losses and loss expenses incurred	829,742,000	601,769,000	707,336,000
Ceded commission earned	268,037,000	241,983,000	201,957,000
Uncollectible reinsurance	$ 946,965	$ 1,010,000	$ 1,049,000