Reinsurance (Reinsurance Financial Information) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reinsurance Disclosures [Abstract]
Written premiums, Direct	$ 7,386,759,000	$ 6,973,216,000	$ 6,726,029,000
Written premiums, Assumed	875,459,000	729,278,000	750,934,000
Written premiums, Ceded	(1,398,719,000)	(1,269,267,000)	(1,216,455,000)
Total net premiums written	6,863,499,000	6,433,227,000	6,260,508,000
Earned premiums, Direct	7,141,427,000	6,851,795,000	6,661,046,000
Earned premiums, Assumed	820,705,000	755,759,000	812,309,000
Earned premiums, Ceded	(1,328,844,000)	(1,236,049,000)	(1,161,936,000)
Net premiums earned	6,633,288,000	6,371,505,000	6,311,419,000
Ceded losses and loss expenses incurred	836,831,000	829,742,000	601,769,000
Ceded commission earned	314,191,000	268,037,000	241,983,000
Uncollectible reinsurance	$ 690,127	$ 946,965	$ 1,010,000