Consolidated Statement of Changes in Partner Capital Statement - USD ($) shares in Thousands, $ in Thousands	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Partners' Capital Account, Units, Redeemed		7,858
Dividends, Preferred Stock, Cash	$ 15,797	$ 15,797
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,600,284	1,908,044	$ 2,082,620	$ 1,936,482
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	105,334	129,004
Other Comprehensive Income (Loss), Net of Tax	22,815	(4,693)
Partners' Capital Account, Contributions	8,675	454
Partners' Capital Account, Distributions	155,844	139,989
Common unit issued as a result of common stock issued by Parent Company, net of purchases	542,514
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures	9,967	$ 10,441
Noncontrolling Interest [Member]
Partners' Capital Account, Units, Redeemed		0
Dividends, Preferred Stock, Cash	0	$ 0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	35,145	30,958	30,486	31,804
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	1,380	1,619
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest	(172)	(127)
Partners' Capital Account, Contributions	8,675	454
Partners' Capital Account, Distributions	5,224	2,792
Common unit issued as a result of common stock issued by Parent Company, net of purchases	0
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures	0	$ 0
Partners Capital Total [Member]
Partners' Capital Account, Units, Redeemed		7,858
Dividends, Preferred Stock, Cash	15,797	$ 15,797
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,565,139	1,877,086	2,052,134	1,904,678
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	103,954	127,385
Other Comprehensive Income (Loss), Net of Tax	22,987	(4,566)
Partners' Capital Account, Contributions	0	0
Partners' Capital Account, Distributions	150,620	137,197
Common unit issued as a result of common stock issued by Parent Company, net of purchases	542,514
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures	9,967	$ 10,441
AOCI Attributable to Parent [Member]
Partners' Capital Account, Units, Redeemed		0
Dividends, Preferred Stock, Cash	0	$ 0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(35,739)	(62,307)	(58,693)	(57,748)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	0	0
Other Comprehensive Income (Loss), Net of Tax	22,954	(4,559)
Partners' Capital Account, Contributions	0	0
Partners' Capital Account, Distributions	0	0
Common unit issued as a result of common stock issued by Parent Company, net of purchases	0
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures	0	$ 0
Limited Partner [Member]
Partners' Capital Account, Units, Redeemed		0
Dividends, Preferred Stock, Cash	0	$ 0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(2,006)	(1,940)	(1,975)	(1,914)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	165	204
Other Comprehensive Income (Loss), Net of Tax	33	(7)
Partners' Capital Account, Contributions	0	0
Partners' Capital Account, Distributions	229	223
Common unit issued as a result of common stock issued by Parent Company, net of purchases	0
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures	0	$ 0
General Partner [Member]
Partners' Capital Account, Units, Redeemed		7,858
Dividends, Preferred Stock, Cash	15,797	$ 15,797
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,602,884	1,941,333	$ 2,112,802	$ 1,964,340
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	103,789	127,181
Other Comprehensive Income (Loss), Net of Tax	0	0
Partners' Capital Account, Contributions	0	0
Partners' Capital Account, Distributions	150,391	136,974
Common unit issued as a result of common stock issued by Parent Company, net of purchases	542,514
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures	9,967	10,441
Partnership Interest [Member]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	105,334	129,004
Other Comprehensive Income (Loss), Net of Tax	22,815	(4,693)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest	$ (172)	$ (127)