In the ordinary course of business, the Company purchases treaty and facultative reinsurance to reduce exposure to significant losses. Facultative reinsurance provides cover for all or a portion of the losses incurred for a single policy and the Company separately negotiates each facultative contract.

Treaty reinsurance provides cover for a specified type or category of risks. The Company's treaty reinsurance agreements provide this cover on either an excess of loss or a proportional basis. Excess of loss covers provide a contractually set amount of coverage after a specified loss amount has been reached. These treaties can provide cover for a number of lines of business within one contract. Under proportional reinsurance, the Company cedes an agreed proportion of the premiums and the losses and loss expenses on the policies it underwrites. These treaties provide us with a specified percentage of coverage from the first dollar of loss.

All of these reinsurance covers provide us the right to recover a portion of specified losses and loss expenses from reinsurers. However, to the extent that reinsurers do not meet their obligations under these agreements due to solvency issues, contractual disputes or other reasons, the Company remains liable. The Company predominantly cedes its business to reinsurers rated A- or better by A.M. Best Company, Inc. ("A.M. Best").

Gross and net premiums written and earned were as follows:




Year ended December 31,	2017						2016						2015
	Premiums written			Premiums earned			Premiums written			Premiums earned			Premiums written			Premiums earned

Gross	$	5,556,273		$	5,616,234		$	4,970,208		$	4,762,394		$	4,603,730		$	4,567,953
Ceded	(1,529,130		)	(1,467,474		)	(1,217,234		)	(1,056,769		)	(929,064		)	(881,536		)
Net	$	4,027,143		$	4,148,760		$	3,752,974		$	3,705,625		$	3,674,666		$	3,686,417

For the year ended December 31, 2017, the Company recognized ceded losses and loss expenses of $1,010 million (2016: $556 million; 2015: $577 million).

At December 31, 2017, the Company's provision for unrecoverable reinsurance was $17 million (2016: $20 million).

At December 31, 2017, gross reinsurance recoverables collectible from reinsurers rated A- or better by A.M. Best were 88.8% (2016: 96.7%).