Commitments and Contingencies (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended									12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Other Commitments [Line Items]
Concentration Risk, Credit Risk, Financial Instrument, Maximum Exposure										$ 1,200,000
Provision for uncollectible premiums receivable		$ 8,000				$ 8,000				8,000	$ 8,000
Rent expense										23,200	$ 33,600	$ 33,000
IT and computer agreements and lease contracts		$ 14,000								$ 14,000
Dividends declared per common share		$ 0.70	$ 0.70	$ 0.70	$ 0.70	$ 0.67	$ 0.67	$ 0.67	$ 0.67	$ 2.80	$ 2.68	$ 2.56
Lease arrangements [abstract]
2016		$ 26,150								$ 26,150
2017		25,312								25,312
2018		12,618								12,618
2019		5,601								5,601
2020		981								981
2021 through 2022		1,398								1,398
Total future minimum rental payments		72,060								72,060
Total future sub-lease rental income through 2019		7,360								$ 7,360
Special Dividend [Member]
Other Commitments [Line Items]
Dividends declared per common share										$ 3.00
Strategic Investments [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
2016		47,000								$ 47,000
2017		35,000								35,000
2018		10,000								10,000
2019		1,000								1,000
2020		0								0
Total		93,000								93,000
Forecast [Member] | Minimum [Member]
Other Commitments [Line Items]
Other Expenses Expected To Be Incurred	$ 30,000
Forecast [Member] | Maximum [Member]
Other Commitments [Line Items]
Other Expenses Expected To Be Incurred	$ 40,000
Structured Letter Of Credit [Member]
Commitments and Contingencies Other Details [Line Items]
LOC obligations guaranteed as of balance sheet date		$ 81,000				$ 61,000				$ 81,000	$ 61,000
Line Of Credit Facility Term										10 years
Maturity date of facility										Dec. 29, 2020