Realized and Net Change in Unrealized Gains, Other Investments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Gain (Loss) on Securities [Line Items]
Total Investment Income	$ 1,176,392	$ 830,527	$ 2,228,040	$ 2,028,967
Other Investments
Gain (Loss) on Securities [Line Items]
Realized Gains	16,207	2,865	16,319	2,870
Net Change in Unrealized Gains	49,643	4,803	45,411	10,291
Total Investment Income	$ 65,850	$ 7,668	$ 61,730	$ 13,161