Reinsurance (Reinsurance Financial Information) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reinsurance Disclosures [Abstract]
Written premiums, Direct	$ 7,874,050,000	$ 7,386,759,000	$ 6,973,216,000
Written premiums, Assumed	973,597,000	875,459,000	729,278,000
Written premiums, Ceded	(1,585,210,000)	(1,398,719,000)	(1,269,267,000)
Total net premiums written	7,262,437,000	6,863,499,000	6,433,227,000
Earned premiums, Direct	7,489,470,000	7,141,427,000	6,851,795,000
Earned premiums, Assumed	941,321,000	820,705,000	755,759,000
Earned premiums, Ceded	(1,499,948,000)	(1,328,844,000)	(1,236,049,000)
Net premiums earned	6,930,843,000	6,633,288,000	6,371,505,000
Ceded losses and loss expenses incurred	955,630,000	836,831,000	829,742,000
Ceded commission earned	358,253,000	314,191,000	268,037,000
Uncollectible reinsurance	$ 7,800,649	$ 690,127	$ 946,965